Roger Federer progressed to the semifinals of the grass-court Gerry Weber Open without playing after Lu Yen-hsun of Taiwan pulled out of their quarter-final with back problems in Halle, Germany, on Friday.

Federer, who began his preparations for Wimbledon by defeating Portugal's Joao Sousa on Thursday, compensated by playing an exhibition against Christopher Kas, a German player who is 78th in the doubles rankings.

The seven-time Wimbledon winner next faces the fourth-seeded Kei Nishikori of Japan, who saved the five break points he faced in a 6-1, 7-6 (4) victory over Steve Johnson of the United States.

Former champion Philipp Kohlschreiber booked his semifinal place with a hard-fought 6-4, 5-7, 7-6 (16) victory over German compatriot Dustin Brown. Brown ousted top-ranked Rafael Nadal of Spain on Thursday.

Kohlschreiber recovered from a 2-5 deficit in the tiebreaker before claiming the win in two hours 10 minutes.

"I don't think a third set ever went so long," the 2011 tournament winner said. "Absolute war of nerves.

"The atmosphere was incredible. But in the end, you just want to leave the court as the winner." 

Kohlschreiber will next face Alejandro Falla of Colombia, who defeated German wild-card entry Peter Gojowczyk 7-6 (4), 7-6 (2). Gojowczyk upset the third-seeded Milos Raonic of Thornhill, Ont., in the second round.