Roger Federer and Nadal through to last four in Qatar

Federer broke twice to take the first set with ease, but then hit two shots long in the tenth 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 tenth 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 No1 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 wasn't surprised by how he played today."

Earlier, Rafael Nadal reached the last four after Belgian opponent Steve Darcis retired with a back problem while trailing 6-1, 2-0. Nadal will next meet Viktor Troicki of Serbia, who beat Lukasz Kubot of Poland 4-6, 6-4, 7-6 (1).

Former world No1s Justine Henin and Kim Clijsters both survived scares before booking their spots in the semi-finals of the Brisbane International.

Henin, playing her first tournament since retiring from the sport in May 2008, looked on track for an easy victory after breezing through the opening set 6-2 against seventh seed Melinda Czink. But the Hungarian, who is ranked 37th in the world, hit back to claim the second.

The third set went the distance, with Czink fighting back from a break down before saving two match points at 4-5 and then forcing a tie-break. But Henin's class and experience prevailed as she closed out the match 6-2, 3-6, 7-6 (7/5) in two hours 20 minutes.

The Belgian will now play third seed Ana Ivanovic in the semis after she defeated Russian Anastasia Pavlyuchenkova 6-2, 7-6 (8/6) .

Clijsters was also made to work hard but eventually edged a rollercoaster clash with Lucie Safarova 6-1, 0-6, 6-4. Clijsters will next face German Andrea Petkovic, who secured her last-four spot with an upset 6-4, 6-2 victory over fourth seed Daniela Hantuchova of Slovakia.