Tennis has a stats problem -- even at the U.S. Open.

The sport often botches even its basic match data, with missing or incorrect numbers for past matches and no archived stats online for individual matches on the women's tour. But at the U.S. Open and the other three Grand Slam tournaments, the events' partner, IBM, has been collecting millions of data points on matches, including stats such as winners and unforced errors that aren't logged at many tour stops. Starting last year, U.S. Open fans have been able to track online...