BASEL, Oct 25 — Stan Wawrinka beat Frances Tiafoe on Thursday at the ATP Swiss Indoors but then revealed that he will not be able to face Roger Federer in the quarter-final due to a back injury.

"The bad news is that I will have to retire,” the 34-year-old three-time Grand Slam winner said an hour after his draining 6-3, 3-6, 7-5 second round victory.

"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. — AFP