Roger Federer overcame Ernests Gulbis' strong serve to reach the Qatar Open semifinals with a 6-2, 4-6, 6-4 victory.
Federer broke twice to take the first set with ease, but then hit two shots long in the 10th game of the second to let the Latvian level the match.
The players traded breaks to make it 3-3 in the third, before Federer capitalised on Gulbis' errors in the 10th game to seal the win.
The 90th-ranked Gulbis finished with 13 aces - seven of them in the second set - and did enough to impress the Swiss No. 1 in their first meeting.
"I know that Gulbis is the player with the most potential right now," Federer said. "Unfortunately, he doesn't have the ranking he deserves. I took him very seriously. I wasn't surprised by how he played today."
In the semifinals on Friday, Federer will play the winner between Nikolay Davydenko of Russia and Ivo Karlovic of Croatia.
Earlier Thursday, Rafael Nadal reached the semifinals after Belgian opponent Steve Darcis retired with a back problem while trailing 6-1, 2-0.
Nadal, a two-time quarterfinalist in Doha, will next meet Viktor Troicki of Serbia, who beat Lukasz Kubot of Poland 4-6, 6-4, 7-6 (1).
"I am playing well," Nadal said. "There was more confirmation that I am hitting the ball well. I started well and served good. I am in the semifinals without losing a set. I am happy with that."
Nadal broke Darcis three times in the first set, and then again in the second game of the final set.
"I wish him a fast recovery," Nadal said. "I hope he is fit for the Australian Open."
AP