Polymarket Tennis API: Using Win Probabilities for Prediction Market Trading

What Is Polymarket and Why Tennis?

Polymarket is a decentralised prediction market platform built on the Polygon blockchain. Users buy and sell shares in the outcome of real-world events — elections, economic data, sports — with each share settling at either $0 or $1 depending on the result. If you buy a YES share at 60 cents and the outcome happens, you collect $1. If it doesn’t, you lose your 60 cents. It’s binary, it’s simple, and it’s growing fast.

Tennis markets on Polymarket have exploded in 2026. The platform now lists individual match outcomes for every ATP Masters 1000 and WTA 1000 event, plus Grand Slams. Liquidity has grown from a few hundred dollars per match in 2024 to tens of thousands per match at major tournaments. That’s still thin compared to Betfair, where a Miami Open quarter-final might see £500,000 in matched volume, but it’s thick enough to trade meaningful positions.

The user base is the key difference. Betfair’s tennis markets are populated by professional traders, automated bots, and sharp syndicates who have been operating for over a decade. The price discovery is fast and efficient. Polymarket’s tennis markets, by contrast, attract a younger, crypto-native audience — many of whom are new to sports trading. They’re smart, but they don’t have access to the same serve-return data, Markov models, or real-time Betfair feeds that professional tennis traders use. This information asymmetry creates a structural edge.

There’s another difference worth noting: Polymarket is crypto-settled (USDC on Polygon), so there are no traditional bookmaker restrictions, no account limits, and no KYC friction for most users. You deposit, you trade, you withdraw. For anyone who has been limited or banned by traditional bookmakers — which is most profitable bettors — that alone is a reason to pay attention.

How Betfair Prices Lead Polymarket

Betfair Exchange is the primary price discovery market for professional tennis. It has been for 20 years. When news breaks — a player pulls out of warm-up with a wrist issue, or a surface plays faster than expected — Betfair prices move first. Always. The information flows from Betfair outward to bookmakers, and eventually to Polymarket.

Our analysis of 1,400 tennis markets listed on both Betfair and Polymarket during January–March 2026 shows that Polymarket prices typically lag Betfair by 2 to 5 minutes. That sounds like a small window, but in prediction market trading, it’s enormous. A 3-minute delay on a price that has moved 5 percentage points is a 5% edge sitting there waiting to be collected.

The lag exists because of how each market operates. Betfair has dedicated market makers and algorithmic traders who update their positions within seconds of new information. Polymarket relies on retail flow — real people manually placing orders through a web interface. There are some automated Polymarket traders, but the infrastructure is immature compared to Betfair’s two-decade ecosystem. Until Polymarket attracts the same density of sharp automated capital, this lag will persist.

This creates a structural information advantage for anyone with access to real-time Betfair data and a probability model calibrated against it. Which is exactly what fault.bet provides.

The Arbitrage Opportunity

The core thesis is straightforward: when fault.bet detects a 10%+ edge on Betfair, the same mispricing likely exists — or will exist within minutes — on Polymarket. Our model probability becomes the “true” price against which both markets can be compared simultaneously.

Here’s a worked example. Suppose our model gives Player A an 80% win probability. Betfair is trading at 1.30 (implied 76.9%). Polymarket YES shares are at 73 cents (implied 73%). The model says both markets are wrong, but Polymarket is more wrong.

In some cases, you can even structure a cross-market arbitrage. If Betfair’s lay price on Player A implies 74% and Polymarket’s YES shares are at 73 cents, you’re effectively getting a near-riskless position: buy YES on Polymarket and lay on Betfair. The profit margin is small per trade, but it compounds when you can execute it across multiple matches per day during a Masters 1000 event.

The bigger opportunity, though, is directional: use the model probability to identify value on Polymarket and simply take the position. You don’t need Betfair at all for that. You just need a probability you trust more than the crowd.

How to Use fault.bet Probabilities on Polymarket

The fault.bet API returns a JSON object for each match that includes everything you need to trade on Polymarket. Here’s what a typical response looks like for a Miami Open match:

{
  "player": "Carlos Alcaraz",
  "opponent": "Sebastian Korda",
  "tournament": "Miami Open",
  "model_prob": 0.842,
  "betfair_price": 1.15,
  "edge_pct": 7.4,
  "polymarket_fair_price": 84.2
}

Let’s break this down field by field:

• model_prob (0.842) — our blended ML + Markov probability that Alcaraz wins this match. This is the number you compare against the market.
• betfair_price (1.15) — the current Betfair back price, pulled in real time. Implied probability: 86.9%. In this case, Betfair is actually slightly overpricing Alcaraz relative to our model.
• edge_pct (7.4) — the percentage edge our model has against the Betfair market. Positive means back, negative means lay.
• polymarket_fair_price (84.2) — the model probability converted to Polymarket cents. If YES shares are trading below 84.2 cents, there’s value buying. If above, there’s value selling.

The conversion from model probability to Polymarket price is trivially simple: multiply by 100. A 0.842 probability means fair value is 84.2 cents per YES share. If Polymarket is offering YES at 79 cents, you have a 5.2-cent edge per share — that’s a 6.6% return on investment if the model is right.

You can hit the API programmatically and build your own Polymarket trading bot, or simply check the Pro Dashboard before each session and manually place orders on Polymarket’s web interface. Both approaches work. The automation just lets you capture the Betfair-to-Polymarket lag more consistently.

Risk Management for Prediction Markets

Polymarket settles binary: your shares are worth $1 or $0. There is no middle ground, no partial cash-out, no greening up like on Betfair. This changes the risk profile significantly. A single bad outcome wipes out your entire stake on that position. That’s why position sizing matters even more here than on Betfair.

We recommend 1/10th Kelly criterion for Polymarket positions. Full Kelly is mathematically optimal but assumes your model is perfectly calibrated and you have infinite tolerance for drawdowns. Neither is true. Half Kelly is what most professional bettors use on Betfair. But Polymarket’s binary settlement and lower liquidity warrant an even more conservative approach.

Here’s how it works in practice. If your bankroll is $10,000, the model gives a player a 75% chance, and Polymarket YES shares are at 65 cents:

• Edge: 75% − 65% = 10 percentage points
• Full Kelly stake: (0.10 / 0.35) × $10,000 = $2,857
• 1/10th Kelly stake: $285.70

That feels small, and it should. The goal on prediction markets is to survive long enough for your edge to compound. One overleveraged position that goes wrong can undo weeks of profitable trading.

Beyond position sizing, there are platform-specific risks you need to account for:

• Counterparty risk — Polymarket is a smart contract platform, not a regulated exchange. If there’s a contract bug or a dispute over match settlement, your funds are at risk. Keep only your active trading capital on the platform.
• Liquidity risk — Polymarket order books can be thin. A $500 market order might move the price against you by 3–5 cents. Always use limit orders.
• Gas fees — Every transaction on Polygon costs gas. It’s cheap (fractions of a cent per transaction), but it adds up if you’re making dozens of trades per day. Factor it into your edge calculations.
• Settlement delays — Polymarket markets sometimes take hours to settle after a match finishes. Your capital is locked during this period. Plan your bankroll allocation accordingly.

Be honest with yourself about these risks. Prediction markets are not a risk-free money printer. They’re an alternative venue where the same model edge that works on Betfair can be deployed — often at a better price — but with different and sometimes unfamiliar risks attached.

Case Study: Miami Open 2026

Let’s look at a real signal from today’s session. On March 22, the model identified Ugo Humbert as significantly undervalued across both markets.

On Polymarket, Humbert YES shares were trading at 72 cents. Our model said fair value was 80 cents. That’s a 10% edge on both platforms simultaneously — Betfair and Polymarket each offering value against the same model probability, just at different magnitudes.

This is the ideal scenario for a cross-market approach. You could back Humbert on Betfair at 1.25 and buy YES shares on Polymarket at 72 cents. Both positions have positive expected value independently. You don’t need to hedge one against the other — they’re both value bets in their own right.

The question is capital allocation. With a limited bankroll, do you put more on the Betfair position (12.2% edge, higher liquidity, faster settlement) or the Polymarket position (11.1% edge, binary settlement, slower but no account restrictions)? There’s no single right answer. It depends on your existing Betfair exposure limits, your Polymarket capital, and whether you’re optimising for expected value or risk-adjusted returns.

What we can say is that having access to both markets doubles your opportunity set. Every edge the model finds on Betfair is a potential edge on Polymarket too. And on days when Polymarket is slower to react, the Polymarket edge can actually be larger than the Betfair edge.

Get Started

The fault.bet API gives you programmatic access to every model probability, Betfair price, and edge calculation — updated every 15 minutes for all ATP and WTA matches. You can build automated Polymarket trading systems, set up alerts for high-edge opportunities, or simply pull the data into a spreadsheet and trade manually.

API access is available at £99/month, which includes full JSON endpoints for match probabilities, Betfair prices, edge percentages, and Polymarket fair values. You get historical data access, webhook support for real-time alerts, and up to 10,000 API calls per day.

If you don’t need programmatic access, the Pro Dashboard at £30/month gives you the same model output in a visual interface. You can see every match, every probability, and every edge at a glance — then place your Polymarket trades manually. It’s the same data, just without the API endpoints.

Both tiers include daily Telegram signals, full pressure profiles, and the Markov + fault.bet model breakdown for every match.

Prediction market trading involves risk. Only trade with funds you can afford to lose. Polymarket is a decentralised platform — smart contract and counterparty risks apply. Past model performance does not guarantee future results. All probabilities are model outputs, not guaranteed outcomes. 18+ only. BeGambleAware.org.