Marat Safin admitted to worries over his form after beating Florent Serra in the Gerry Weber Open first round.
Safin felt out on sorts in his Halle opener
The former world number one beat the 2005 finalist 7-6 (7-4) 6-3 but said: "I'm still struggling with confidence.
"At the start, I was missing my rhythm. It is difficult playing my first match on grass to find a way. But I'm feeling pretty fit - my knee doesn't hurt."
Second seed Nicolas Kiefer pulled out of the event in Halle due to a recurring wrist injury.
"Since arriving in Germany I have not been able to train without pain," said the German who pulled out of the French Open at the second round stage due to the same problem.
Kiefer won the event in 1999 and reached the final in 2002 and 2003.
He must now be a doubt for Wimbledon in two weeks time.
France's Fabrice Santoro, a finalist in 2001, beat Mikhail Youzhny of Russia 7-6 (7-4) 6-2.
Compatriot Richard Gasquet also advanced, seeing off Swiss qualifier George Bastl 6-2 6-3.
The 19-year-old served nine aces and hit 17 winners to set up a potential match with Roger Federer, who plays Indian qualifier Rohan Bopanna on Wednesday.
Federer is chasing his fourth straight Gerry Weber title ahead of his Wimbledon title defence.