Roger Federer progressed to the semi-finals of the grass-court Gerry Weber Open without playing after Lu Yen-hsun of Taiwan pulled out of their quarter-final with back problems.

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 semi-final place with a hard-fought 6-4, 5-7, 7-6 victory over German compatriot Dustin Brown.

Brown ousted world number one 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, 7-6. 

American Steve Johnson was playing the fourth-seeded Kei Nishikori in the last quarter-final.

Source: AP