Jack Sock claimed the biggest victory of his career Friday, upsetting seventh-seeded Marin Cilic, 6-4, 6-3, 6-3, to advance to the fourth round of the United States Open for the first time.
Sock, a 23-year-old from the Kansas City area, has been a leader of the next generation of American men since winning the United States Open junior championship in 2010.
But so far he has been most successful in doubles. He has Grand Slam titles in doubles and mixed, and won medals in doubles and mixed doubles at the Rio de Janeiro Olympics. But he split with his longtime partner Vasek Pospisil to focus on singles.