The Swiss veteran, a 17-time Grand Slam winner, is seeking a ninth title in the Wimbledon warm-up event in Halle, and fourth on the trot on the German grass.
Top seed Federer will play Alexander Zverev in Saturday's last four after the young German beat Cypriot Marcos Baghdatis 7-6 (11/9), 6-3 earlier on Friday.
Austria's Dominic Thiem, the No.3 seed, is also through to the semi-finals after German Philipp Kohlschreiber pulled out of their quarter-final.
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