Answer
Roger Federer is the only player that Justin Gimelstob did not defeat in singles. Gimelstob had a successful career, reaching a career-high ranking of No. 6 in the world and winning 11 ATP singles titles. However, he never managed to defeat Federer, who is widely considered one of the greatest tennis players of all time. Federer has won 20 Grand Slam singles titles and has been ranked No.