Indoor tennis: are favourites more reliable under a roof?

No wind, no sun in your eyes, a bounce that doesn’t change from one day to the next. Indoors, tennis is played in stable conditions, and the conclusion seems obvious: the better player should win more often there. You also hear about “indoor specialists”, players who supposedly shine under a roof more than anywhere else.

The end of the season is the right time to check: from October to November, almost the whole men’s tour moves inside, from Basel and Vienna to the Rolex Paris Masters and the season-ending Finals. We went back over 37,686 ATP and WTA singles matches played between January 2022 and September 2026, including 2,565 on indoor hard courts, and compared each result with what both players’ Elo ratings predicted.

The favourite wins as often indoors as outdoors

To isolate the effect of the roof, we compare indoor hard courts with outdoor hard courts: same surface, only the conditions change. Clay and grass are shown for reference.

64.9% indoors, 65.0% outdoors: no difference at all. The favourite even wins slightly more than expected indoors (0.9 points), but a gap that size happens by chance more than one time in three. The surface that really trips up favourites is still grass, where they win 2.2 points less than expected, as our study on surfaces showed.

Even for heavy favourites

You might think a roof mostly helps clear favourites, the ones nothing should disturb. It doesn’t: when Elo gave the favourite at least an 80% chance, they won 82.3% of their matches indoors (345 matches), against 84.0% on outdoor hard courts. The tour doesn’t change the conclusion either: on the ATP, the favourite wins 1.6 points more than expected indoors, on the WTA 2.8 points less (only 429 matches), two gaps that can’t be told apart from chance.

Paris Masters: no more upsets than elsewhere

The Rolex Paris Masters has a reputation as a trap: end of season, worn-out players, Finals qualification at stake. Over its last four editions (293 matches in our sample), the Elo favourite won 64.5% of matches, against 63.9% expected. In the other hard court Masters 1000 events, played outdoors (Indian Wells, Miami, Canada, Cincinnati, Shanghai), the favourite won 62.8% against 64.8% expected (1,889 matches). Paris is no lottery, and 293 matches are not enough to say it is more predictable than the others.

Predictions for the 2026 edition, and the model’s record on past editions, are on our Paris Masters predictions page.

Indoor specialists are nowhere to be found

That leaves the idea of the specialist. To test it, before each indoor match we looked at how the player had performed under a roof so far, compared with their Elo.

A first, misleading reading: players who had beaten their Elo by at least 6 points over fifteen or more previous indoor matches keep beating it in the next one, by 3.9 points. But the same calculation outdoors also shows a lasting edge (1.8 points). What this measures is mostly a player on the rise: Elo catches up with them late, and they beat their predictions everywhere, not just indoors.

To remove that effect, we defined a specialist as a player whose edge indoors exceeds their edge on outdoor hard courts by at least 6 points, over at least fifteen matches in each. The result:

Both groups beat their Elo, almost identically: they are main-tour regulars who have already played at least fifteen matches in each condition. The gap between them, 1.4 points, can’t be told apart from chance, and it ranges from 1 to 5 points as soon as the thresholds change. In our data, being good indoors mostly means being good.

The only hint: the odds

One number does stand out. Across the 1,001 indoor matches for which we have bookmaker odds, the market favourite won 69.7% of matches, while the odds gave them on average a 66.2% chance once the margin was removed. Outdoors, the gap is only 0.7 points (67.5% against 66.8%, 8,297 matches).

The market therefore seems to slightly underrate favourites indoors. The gap points the same way on the ATP and the WTA, before 2025 and after. But the difference with outdoor matches (2.9 points) is on the edge of chance: it would show up about one time in sixteen with no real effect at all. And the odds you actually bet on include the bookmaker’s margin, which our calculation removed. It is a hint worth watching, not a strategy: our betting guide explains why gaps like this rarely survive real stakes.

How to read an indoor match

• Don’t rate the favourite higher because the match is under a roof. At equal level, they win indoors as often as outdoors.
• Be wary of the “indoor specialist” label. A good indoor record mostly reflects the player’s overall level, which is already in their Elo.
• Don’t treat the Paris Masters as a lottery. Favourites win there as often as expected.
• Look at what really matters instead: the gap in level, and a possible return after a long break.

The Pronocast model knows whether a match is played indoors: it is one of the variables it receives, along with Elo, form and the odds. This study shows that, on its own, it barely changes who the favourite is. To see the predictions every day, open today’s predictions, and the verified results page shows what they are worth once the matches are played.

Method

ATP and WTA singles matches from 1 January 2022 to 24 September 2026, where both players had already played at least twenty matches. The indoor flag comes from our data provider, which got several tournaments wrong: we removed Budapest (WTA, actually outdoor clay), the 2023 Asian Games (outdoors), Tokyo (WTA, uncertain), Linz 2026 (clay), plus the Laver Cup and the Next Gen Finals, whose format differs. That leaves 37,686 matches, including 2,565 on indoor hard courts and 17,684 on outdoor hard courts. The expected rate is the actual win rate observed for a given Elo probability, in 2.5-point bands, across the whole sample. Gaps are judged with a z-test. For specialists, only previous matches count, never the match being measured. Odds are the bookmakers’ average, converted to a margin-free probability, available for 9,304 hard court matches since August 2022. The full script is kept with our data.