Roger Federer began his bid for a ninth Gerry Weber Open title with a 6-4 7-6(3) first-round win over German wildcard Jan-Lennard Struff on Thursday.
The Swiss top seed, still without a title during an injury-hampered 2016 season, kept his cool during a contest that featured only one break of serve.
Federer, who missed the French Open due to a back problem, fired 15 aces en route to setting up a last 16 encounter with Tunisia's Malek Jaziri.
Second seed Kei Nishikori's title bid at the Wimbledon warm-up tournament ended in the second round after he pulled out with a rib injury before his second-round match against Germany's Florian Mayer.
"I'm very sad to not be able to go on playing," Japan's Nishikori, who needed treatment during his first-round win against Lucas Pouille, told the Halle tournament's official website.
Nishikori said he was confident the injury would improve before the start of Wimbledon on June 27.