Nadal has been battling with a fever in Qatar
Top seed Rafael Nadal booked his place in the last four of the Qatar Open but was made to work hard by Latvia's Ernests Gulbis in the quarter-finals.
Nadal won the first set on a tie-break after surrendering a 4-1 lead before sealing a 7-6 (7-3) 6-3 victory.
Spain's world number one will take on defending champion Nikolay Davydenko, who beat Ivo Karlovic 6-3 7-5.
Earlier, Roger Federer secured a semi-final clash with Jo-Wilfried Tsonga after beating Viktor Troicki 6-2 6-2.
Nadal took advantage of an error-strewn performance from his 22-year-old opponent - the Spaniard making 11 unforced errors compared to 31 from Gulbis - to make the last four.
The world number one broke his opponent in the fourth game of the first set and although Gulbis broke back, the Latvian could not maintain his form in the tie-break.
Nadal then broke twice more in the second set to secure victory in one hour and 51 minutes.
"Every match is really tough," he said. "Gulbis is one of the best players in the world and he has a great opportunity to go up and be in the top 10. He is very dangerous.
"I started really well and the first five games was the best I have played this week. But I went down and he started to play better.
"But after that I played a solid match."
Federer took only 65 minutes to see off his opponent and set up a last-four clash with Frenchman Tsonga, who defeated Guillermo Garcia-Lopez.
The 29-year-old Swiss broke Troicki twice in each set while holding serve easily against the world number 28, who claimed his first ATP Tour title
in Moscow last October.
Federer, winner of 16 Grand Slam titles, did not have to save a break point as he kept up his record of not dropping a set in the tournament.
"It wasn't a tough match and I was quite comfortable," reflected Federer, a two-time winner in Doha.
"I am happy with my form here and in this match. I was in control of my shots and created many opportunities. This was easy compared to the first two matches."
Tsonga, who is making his comeback from a knee injury that curtailed his 2010 campaign, defeated seventh-seeded Garcia-Lopez 7-6 (7-4) 7-6 (11-9).
The world number 13 from France, who reached the
Australian Open final in 2008,
beat Federer in the quarter-finals of the
Montreal Masters in 2009,
lost to the Swiss in the semi-finals
of the 2010 Melbourne Park Grand Slam tournament.
"He's one of the game's great characters, and a dangerous player," Federer added of Tsonga. "He has performed well at the big tournaments, especially on the hard surface at the Australian Open."