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American Jack Sock had to retire against Ruben Bemelmans after he collapsed onto the court amid the intense heat at the U.S. Open on Thursday. Al Bello/Getty Images North America/Getty Images
Sock, seeded 28th, led the match two sets to one. He also retired last year at the U.S. Open due to a calf injury. Alex Goodlett/Getty Images North America/Getty Images
Andy Murray, the champion in 2012, fell behind two sets to Frenchman Adrian Mannarino before cruising in the next three. Clive Brunskill/Getty Images North America/Getty Images
Stan Wawrinka, like Murray a two-time grand slam champion, was tested by South Korean teen Hyeon Chung but advanced in three tiebreaks. Elsa/Getty Images North America/Getty Images
Murray has appeared in the semifinals, at least, in all three majors this season. Streeter Lecka/Getty Images North America/Getty Images
Victoria Azarenka, a two-time U.S. Open finalist, won in two sets against Yanina Wickmayer. Clive Brunskill/Getty Images North America/Getty Images
Many consider Azarenka to be Serena Williams' biggest threat in New York. Williams is attempting to win all four majors in one season. STAN HONDA/AFP/Getty Images
Garbine Muguruza, who made the Wimbledon final and lost to Williams, was upset by qualifier Johanna Konta. Muguruza, pictured, cut ties with her coach last week. Matthew Stockman/Getty Images North America/Getty Images
Sam Stosur, the 2011 winner, spent less than an hour on court in defeating Evgeniya Rodina. But she criticized tournament organizers for the way they handle transportation. Elsa/Getty Images North America/Getty Images
