Tommy Haas lined up a second-round meeting with Roger Federer, while seeded players Gilles Simon and Viktor Troicki both made an early exit from the Stuttgart Open on Tuesday.
The 39-year-old Haas, playing for the last time at the grass-court tournament, defeated Pierre-Hugues Herbert 6-3, 4-6, 7-5 in their first-round match. The victory set up another clash with Federer, who has already won 15 titles on grass and is playing again after a 10-week break.
"I'm really looking forward to playing the next game against one of my closest friends," Haas said. "We had many great battles in the past. He is the best player on grass in history. I'll definitely need to play better to have a chance. But you never know what can happen."
Federer hasn't played a tournament since winning the Miami Open in early April, skipping the entire clay-court season to recuperate after a busy start to the year that saw him capture his 18th Grand Slam title at the Australian Open.
He said Monday in Stuttgart that he would play a full schedule for the remainder of the season.
"There are no more breaks now," he said in an interview with the ATP Tour. "I've had enough breaks. I'm a practice world champion now and that's not who I want to be. I want to be a champ on the match courts. So I'm going to be playing a regular schedule for the second part of the season."
Feliciano Lopez of Spain upset the seventh-seeded Simon 6-3, 6-3, while Benoit Paire of France beat the eighth-seeded Viktor Troicki 6-4, 6-4.
German qualifiers Peter Gojowczyk and Yannick Hanfmann both advanced, as did fellow countryman Philipp Kohlschreiber, who was leading 6-1 when Marcos Baghdatis retired.
Another German, Florian Mayer, later lost 7-6 (3), 4-6, 7-6 (1) to Jeremy Chardy of France, who next plays Lopez.
Jerzy Janowicz of Poland defeated Russia's Andrey Kuznetsov 6-4, 3-6, 6-3, and Bernard Tomic prevailed over Stephane Robert of France, 6-1, 5-7, 6-2.
The top four seeded players - Federer, Grigor Dimitrov, Tomas Berdych and Lucas Pouille - received byes to the second round.
This is the third year the event is being played on grass.