Second seed Kei Nishikori survived a first-set scare to defeat Lucas Pouille at the Gerry Weber Open, but David Ferrer was an opening-round casualty on Monday.
Nishikori, a two-time semi-finalist in Halle, had a mixed 24-18 record on grass before the encounter in Germany and things appeared bleak when he lost the first set on a tie-break.
However, the Japanese showed a marked improvement from there on in and he struck 24 winners en route to securing a 6-7 (5-7) 6-1 6-4 victory.
There was no such luck for Spaniard Ferrer, though, as the sixth seed was beaten 6-3 6-4 by Andreas Seppi.
Seppi was a beaten finalist by eight-time champion Roger Federer 12 months ago and he once again showed form on the German grass.
It was also a tough afternoon for fifth seed David Goffin, who needed three sets to defeat NextGen star Borna Coric 7-6 (7-5) 6-7 (3-7) 6-4.
In other matches, Sergiy Stakhovsky defeated Denis Kudla 7-5 4-6 6-3 and Benjamin Becker gave the home fans something to cheer with a 7-5 6-3 beating of Ernests Gulbis.
Yuichi Sugita and Taylor Fritz were locked at 6-6 in their first-round match, which will be completed on Tuesday.