DOHA (Reuters) - Flu victim Rafa Nadal benefited from a first set timeout with the doctor to dispatch Latvian Ernests Gulbis and join old foe Roger Federer in the semi-finals of the Qatar Open on Thursday.

Nadal, who has been suffering from a fever all week, began brightly by breaking Gulbis for a 3-1 lead in the opening set but lost his serve and called for the doctor after complaining that he was sweating more than normal on a pleasant evening in Doha.

The stoppage helped the world number one, who recovered and took a narrow tiebreak before wrapping up a 7-6 6-3 victory to set up a meeting with either Russian Nikolay Davydenko or big serving Ivo Karlovic for a place in the final.

Despite feeling under the weather, Nadal produced a typical energetic display but was helped by an unpredictable Gulbis forehand, which swung from crisp to crusty throughout the match and contributed to many of his 31 unforced errors.

Earlier, Federer looked back to his scintillating best in blitzing Serbian Viktor Troicki 6-2 6-2 in his quarter-final after two average performances.

The 16-times grand slam champion took only four of the 16 break points opportunities he created on Troicki's serve but it was more than enough to complete the straightforward victory.

"I thought it was very concentrated performance," Federer said.

"I really thought I saw the ball a bit better. From my side, I'm really happy the way I was able to keep Troicki on the back foot, make him feel like he didn't know what was coming his way."

Federer will play third seed Jo-Wilfried Tsonga for a place in the final and the Swiss said he was looking forward to the match with the Frenchman, who is in his first tournament back following a knee injury.

"First of all, it's nice to see him back," Federer said. "It's never nice to miss those guys because of injuries. I think it's exciting playing him again. I think he's a very explosive player with good character, good for the game. Looking forward to the match, it's going to be difficult."

(Reporting by Patrick Johnston; editing by Justin Palmer)