Second seed Kei Nishikori of Japan has pulled out of the Halle ATP event in Germany with a rib injury.
He had reached the second round of the traditional Wimbledon warm-up tournament and was due to face German Florian Mayer on Wednesday but told organisers he was unable to play.
Mayer advanced to the quarter-finals with a walkover after a first-up 7-5 7-6 win over Brian Baker of the US.
Nishikori took a medical timeout during his first-round win over Lucas Pouille.
Elsewhere in the second round, local eighth seed Philipp Kohlschreiber beat Marcel Granollers of Spain 6-2 7-6 (7-4).
In earlier opening-round action, Marcos Baghdatis beat Tomas Berdych 7-6 (7-3) 7-6 (7-4), taking a measure of revenge for his loss in the 2007 final.
Rising prospect Alexander Zverev downed seventh-seeded Viktor Troicki 6-4 6-4, while Ivo Karlovic pounded 26 aces to move past Andrey Kuznetsov of Russia 7-6 (8-6) 6-3.
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