Basel: Stan Wawrinka was forced to withdraw from Friday's Basel quarter-final showdown against best friend Roger Federer with a back injury, just an hour after struggling past America's Frances Tiafoe in a gruelling second round clash. "The bad news is that I will have to retire," the 34-year-old three-time Grand Slam winner said after his 6-3, 3-6, 7-5 win. "I had some trouble with my back in the last game. I'm sure I cannot make it for tomorrow (Friday). I will pull out." Top seed Federer, chasing a 10th title at his hometown event, will receive a walkover into Saturday's semi-finals.

It would have been the 27th meeting between the two Swiss teammates and Federer would have been the comfortable favourite boasting a 23-3 record over their 14-year rivalry. Wawrinka stands in provisional 14th place in the points race to the year-end ATP Finals and his chances of qualifying now look slim with just next week's Paris Masters to play on the regular schedule.