Swiss Indoors: Stan Wawrinka pulls out of match with Roger Federer with back injury
Stan Wawrinka withdrew from his Swiss Indoors quarter-final with nine-time champion Roger Federer because of a back injury.
The 34-year-old seventh seed, who lost to Andy Murray in last week's European Open final, beat Frances Tiafoe in two hours 32 minutes in Basel.
But an hour after his 6-3 3-6 7-5 win he announced his withdrawal.
Fellow Swiss Federer is seeking his 10th title and receives a walkover into Saturday's semi-finals.
In his first meeting with world number 48 Tiafoe, who beat British number one Dan Evans in the previous round, Wawrinka served 14 aces and claimed the decisive break to win the match after a marathon 12th game containing six deuces.
But the world number 17 said: "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."
Third seed Stefanos Tsitsipas beat Ricardas Berankis 6-7 (4-7) 6-2 6-4 to reach his 13th quarter-final of the year.
Greek Tsitsipas, 21, won in two hours one minute and next plays Serbian world number 46 Filip Krajinovic, who beat fifth seed Fabio Fognini 6-2 6-4.