Roger Federer came through his opening round match at the Gerry Weber Open on Wednesday, while Kei Nishikori pulled out with a rib injury.
Top seed Federer, seeking his ninth title in Halle, beat German wildcard Jan-Lennard Struff 6-4 7-6 (7-3) to progress as he steps up his preparation for Wimbledon after missing the French Open due to a back injury.
Federer, beaten by Dominic Thiem in the semi-finals of Mercedes Cup last week, only converted one of four break point in the match, but that was enough to see him through.
Florian Mayer was given a walkover to the quarter-finals after second seed Nishikori was unable to take to the court.
"I'm very sad to not be able to go on playing," said the Japanese player, who needed a medical timeout during his opening round victory against Lucas Pouille.
However, the 26-year-old insisted he will be fit enough to compete at Wimbledon, while Mayer will next face Andreas Seppi, who beat Yuichi Sugita in 6-3 6-0 in under an hour.
Alexander Zverev, the 19-year-old who knocked out seventh seed Viktor Troicki in the first round, booked a place in the quarter-finals after fellow German Benjamin Becker withdrew with a back injury during the second set of their match after losing the first.
Dustin Brown was unable to celebrate receiving a Wimbledon wildcard after losing in straight sets to Marcos Baghdatis, who saw off fourth seed Tomas Berdych in the first round.
Philipp Kohlschreiber, winner of the Halle tournament in 2011, is through to the second round of his home event after knocking out Marcel Granollers in straight sets.
Third seed Thiem and Teymuraz Gabashvili also won their first round matches on Wednesday.