After every few dashes across Court 14, Kelly Bruno reached down to her right leg and flicked at something. It was a gesture so slight and so fleeting, she could have been swatting away a bug. It was also the only thing she did that was not in the protocol for a United States Open ball girl — nowhere does it mention popping the pressure valve on a prosthetic leg.

Bruno was born with several defects in her left leg and has been an amputee since she was 6 months old. By 18, she had turned herself into a track star among disabled athletes, with her own sponsorship deals. And as a world-class triathlete and Ironman competitor, she has raced in some of the most grueling events on the planet.

Now, at age 24, Bruno has scaled back her training for three weeks to shuttle back and forth across courts at the Billie Jean King National Tennis Center to scoop up balls at the net. In a job done correctly only by those who are barely noticed, Bruno has stood out by blending in.