Win rate is the most-quoted stat in amateur tennis and the most misread. A 70% win rate over 10 matches against the same opponent means almost nothing. A 55% win rate over 80 matches against varied competition means a lot.

Sample size beats percentage

Statisticians generally consider 30 matches the minimum for a meaningful win-rate baseline. Below that, single matches swing the number wildly. Tennis Log shows a confidence band on your trend chart for exactly this reason — wide early, narrow as you log more.

Rolling averages vs cumulative

Cumulative win rate: every match you've ever played. Stable, but slow to react to recent form.
Rolling 10-match win rate: the metric to actually watch. It shows current form and surfaces hot/cold streaks.

What a healthy curve looks like

A genuinely improving player shows a gradually rising rolling average with shrinking volatility. Spikes that immediately collapse are usually opponent-strength artifacts, not skill changes.

Red flags in the trend

Sharp drops that align with switching surfaces or opponents
Improvements only against one specific player
Win rate rising while hold % is flat (you're getting lucky on returns, not improving on serve)

Pair your win rate trend with opponent strength and surface filter to see whether you're actually getting better or just playing easier matches.