Andre Agassi slumped to a 7-5, 7-5 defeat against Germany's Bjorn Phau on Wednesday in the second round of the Dubai Open, only his second tournament of 2006.
A passive Roger Federer struggled before defeating Kuwait's Mohammed Al Ghareeb 7-6 6-4 to reach the quarter-finals.
Russia's Marat Safin failed to follow up his first round victory over world number five Nikolay Davydenko, losing 7-6 6-4 to Belgian Olivier Rochus in his first tournament since a six-month injury break.
A 7-5, 2-6, 7-6 victory over seventh-seeded Czech Radek Stepanek earned Briton Tim Henman a meeting with Spanish teenager Rafael Nadal.
Agassi, 35, looked second best against his 80th-ranked opponent who played solidly from the baseline and showed astonishing speed to the net.
"The ball was really flying tonight and I never got comfortable with my range," Agassi told reporters.
"I felt like I couldn't make three or swings in a row, and against a guy that quick I started pressing with the first couple of shots. Once I allowed him to use his legs it was pretty much an uphill battle from there.
"I needed to settle and start making a few, but as soon as I did that I felt he was taking his chances and he never let me settle in."
Phau's aggressive play allowed the German to break for 3-2 in the first set.
Agassi levelled at 4-4 when Phau netted a forehand, but the German struck a glorious backhand pass on the run to break again for 6-5 and he served out the set to love.
Both players earned break point opportunities in the second set and it was Phau, nine years Agassi's junior, who broke first to lead 4-3.
Agassi again drew level at 4-4, held for 5-4 and held two set points which Phau saved with an ace and by forcing Agassi into a forehand error.
AGASSI LEGEND
Another forehand error gave Phau a decisive break for 6-5, and a fine serve allowed him to hold off a break point in the next game as he served out the match.
"I just tried to hang in the match and after I won the first set I definitely believed I could win it," said Phau.
"Agassi is a legend. I watched him on the tv as a little kid and it's definitely the biggest win for me.
"My serve played a great role for me, which is not normal. And my speed is the main weapon in my game.
Serving well against Federer, Al Ghareeb - ranked 488 in the world - showed no sign of nerves in the biggest match of his career and after breaking to lead 3-2, he consolidated his advantage by firing four aces past the world number one.
The Kuwaiti fought off four break points to lead 5-3 and came within two points of claiming the set before Federer edged the tiebreak 7-5.
The Swiss also found it difficult to take control of the second set before Al Ghareeb, 25, double-faulted to give Federer a 4-3 lead on his eighth break point of the set.
"I was hoping it was going to be easier, obviously, but he played well. He definitely served well, and I was playing quite passive," said Federer.
"One I got tied up in that passive play I couldn't really play my forehand and it was getting hard. I was lucky I got back in that first set.
"He was the better player but my experience got me through."
