Marat Safin and Andre Agassi were high-profile casualties in the second round of the Dubai Open on Wednesday.
Safin was playing his first tournament since August
Safin's return from a six-month injury lay-off came to an abrupt end with a 7-6 (7-5) 6-4 defeat by Olivier Rochus.
Agassi - also making his way back after injury problems - could not build on his opening win over Greg Rusedski and went down 7-5 7-5 to Bjorn Phau.
Top seed Roger Federer earlier had to dig deep to beat Kuwaiti wild card Mohammed Al Ghareeb, winning 7-6 6-4.
The world number 488 had Federer in trouble when he served for the first set but could not convert and the Swiss star pulled through.
"I hoped it would be easier obviously," said Federer. "I was playing quite passively and got caught up in that. I guess my experience got me through."
Safin impressed with victory over Nikolay Davydenko on Tuesday but lost a tight first set against Rochus, who needed three breaks in a see-saw second set to get the win.
"It was a lack of matches," said Safin, who threw his racket to the ground in frustration three times.
"You need to get the rhythm back, it doesn't come straight away. There's still a long way to go and many things to work on.
"It's not like I was so bad but I never like to play him. He's a difficult opponent because he uses slice and changes his game a lot. He plays smart. I just need matches."
Agassi made an unusually high 31 unforced errors in a disappointing effort against world number 79 Phau.
"The ball was really flying tonight," said Agassi. "I never got comfortable with my range. I couldn't make three or four shots in a row.
"I was pressing with the first couple of shots but once I let him use his legs it was a pretty uphill battle after that.
"It feels like I was playing tennis with a golf ball and it was not a comfortable feeling at all."