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.