In world of elite tennis, not everyone believes rivals can be trusted to make fair calls in umpire-free practice sessions

During a calm, quiet afternoon in the buildup to Wimbledon last year, the former US Open champion Sloane Stephens was working on her game in a practice set against another player when her opponent’s mood suddenly soured. Stephens prides herself on her integrity and, having contested hundreds of practice matches over the years, she says she always veers on the side of caution when charged with calling her own lines. Only when she is certain there is a clear gap between the ball and line does she call an opponent’s shot out.

Halfway through the set, Stephens believed her opponent had missed a second serve and called a double fault. “She thought I cheated her in the practice,” says Stephens, smiling. “I didn’t know this happened [during] the practice. I was like: ‘Second serve out, double fault.’ Apparently, she looked back at her coach in disgust.”