WIMBLEDON, England — Serena Williams expressed no qualms about antidoping officers showing up unannounced to collect urine and, on occasion, blood samples. She wasn’t even bothered when it happened twice in the same week in the lead-up to this year’s French Open. It was like T.S.A. searches at the airport — a minor inconvenience of her high-flying tennis career.

The five out-of-competition tests in the first six months of 2018 didn’t irritate Williams until she saw numbers, plucked from the United States Anti-Doping Agency’s public database and included in a recent Deadspin article, that seemed to suggest she was being tested far more often than her compatriots in the sport.

The registered pool of performers included the United States Open champion Sloane Stephens and the finalist she beat, Madison Keys, who were listed as having one completed test, and Sam Querrey, the men’s world No. 13, who had none.

“I didn’t know I was being tested three times more — in some cases five times more — than everyone else,” Williams said Monday after her straight-sets victory over Arantxa Rus in the first round at Wimbledon.