Elo rankings in tennis, explained
How tennis Elo works, why it sometimes disagrees with the ATP and WTA, and what it gets right.
What Elo measures
Elo is a paired-comparison rating system: every match shifts both players' ratings based on the result and on how surprising it was. A 1,500-rated player who beats a 1,800-rated player gains a lot. The same player beating a 1,200-rated opponent gains very little.
Why it disagrees with the ATP/WTA
Tour rankings reward turning up: a player who plays 30 events and reaches the quarters of all of them will out-rank a player who plays 18 events and wins six of them. Elo does the opposite — it rewards quality of opposition and result, not volume. That is why younger or part-time players sometimes have a much higher Elo than their official rank.
Surface-specific Elo
Most serious tennis Elo lists are split by surface — hard, clay, grass — because the player who is best on grass is rarely the player who is best on clay. Look at the surface-specific Elo before betting on a clay-court Masters event.
Caveats
Elo cannot model injuries, fatigue, or a player who has just changed coaches. It is one number among many — a sanity check, not the final word.