Canadian Milos Raonic has undergone hip surgery in Vail, Colorado, and is not expected to return to practice for another six weeks, putting his participation in the U.S. Open at serious risk. Raonic suffered the injury when he took a nasty spill in the second round of Wimbledon against Gilles Muller and was forced to retire. The Canadian Tennis Association said that he is already back riding a stationary bike. The 20-year-old Raonic made a splash earlier this year by reaching the fourth round of the Australian Open and winning San Jose. He's currently ranked No. 26.