A back injury hampered the progress of Stanislas Wawrinka, who was forced to pull out of his Basel quarter-final clash against Roger Federer. Wawrinka aggravated his back during his 6-3 3-6 7-5 win over American Frances Tiafoe. And as a result, Federer, a nine-time winner received a walkover and advanced to the semifinal.

“The bad news is that I will have to retire,” Wawrinka, a 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.”

Friends Federer and Wawrinka have battled each other 26 times in the past with the 20-time Grand Slam winner leading an overwhelming 23-3. Federer will face either Stefanos Tsitsipas or Filip Krajinovic on Saturday and should he win to reach the final, the 38-year-old will leapfrog Russia’s Daniil Medvedev to the third spot in race to the year-end ATP Finals.

Wawrinka on the other hand is currently placed 14th and his chances look slim with only next week’s Paris Masters to go. Wawrinka and Tiafoe played out a gruelling match which lasted over two and a half hours fetched the Swiss his fifth Basel quarter-final berth. He delivered 14 aces in the game. Tsitsipas was equally impressive as he walked out to a round of applause from 90000 people.

“He was giving me a hard time in the first set, it was not easy to overcome, we had a high-quality match. I managed to remain calm and not rush too much,” World No. 7 Tsitsipas said.