HALLE, Germany -- Roger Federer progressed to the semifinals of the grass-court Gerry Weber Open without playing after Yen-Hsun Lu of Taiwan pulled out of their quarterfinal with back problems 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.
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 world No. 1 Rafael Nadal on Thursday.
Kohlschreiber, who won the tournament in 2011, will next face Alejandro Falla of Colombia, who defeated German wild card Peter Gojowczyk 7-6 (4), 7-6 (2).
American Steve Johnson was playing the fourth-seeded Kei Nishikori in the last quarterfinal.