Federer won his first grass-court game of the season 7-6 (7/1), 6-3 on a muggy evening in Halle to set up a second-round meeting with French veteran Jo-Wilfried Tsonga.
“It is always important to win your first game on grass, because otherwise the grass season can be very short,” said Federer.
Tsonga, 34, said it would be “something special” to face Federer after he beat compatriot Benoit Paire 6-4, 7-5 on Tuesday.
The 30-year-old Paire, infamous for his on-court tantrums, was given a warning in the first set after throwing his racket, but had the crowd laughing in the second when he and Tsonga engaged in a spontaneous keepy-uppy rally.
“I am happy to have won because Benoit is a dangerous player. It is always difficult to dominate against him, you always feel like the game can change at any moment,” Tsonga told AFP.
The Frenchman faces Federer on Thursday, and can boast of a perfect record against the Swiss on grass, having won their only meeting on the surface in 2011.
“I used to live nearby. I would come here every year and lose, so it is great to have finally won,” said Struff.
There was to be less joy for the Halle crowd as Germans Mats Moraing, Peter Gojowczyk and Rudolf Molleker were all knocked out on Tuesday
Zverev, who hurt his knee during a first-round win over Robin Haase on Monday, remains in the singles draw and is due to face American Steve Johnson on Thursday.
Moraing lost 4-6, 6-7 (4/7) to Italy’s Andreas Seppi while Ukrainian Sergiy Stakhovsky came from behind to beat Molleker 3-6, 7-6 (8/6), 6-2.
Gojowczyk was crushed 6-3, 6-4 by France’s Richard Gasquet.
Gasquet will face seventh seed Roberto Bautista Agut in the second round after the Spaniard cruised to a 7-6 (12/10), 6-0 victory over American Taylor Fritz.