World number one Roger Federer struggled over three sets, while number two Rafael Nadal had an easy sail as both reach the Qatar Open semi-finals on Thursday. Federer survived a scare to down 21-year-old Latvian Ernests Gulbis winning 6-2, 4-6, 6-4 to set up showdown against Russian third seed Nikolay Davydenko who beat big-serving Croat Ivo Karlovic 7-6, 7-6.

Nadal stayed in shape when Belgium's Steve Darcis retired with an injured back when the Spaniard was leading 6-1, 2-0 in their collision.

Spain's Rafael Nadal serves to Potito Starace of Italy during the ATP Qatar Open tennis tournament in Doha January 6, 2010.

Nadal will next play Serbia's Viktor Troicki who overcame Poland's Lukasz Kubot 4-6, 6-4, 7-6. "This wasn't the way I wanted to reach the semi-finals. I feel sorry for Darcis and wish him a speedy recovery. I hope he plays in the Australian Open. When you know you aren't well, it's better to stop," said Nadal.

The Spaniard said he was back to his best after being plagued by knee and stomach injuries during 2009.

Switzerland's Roger Federer watches a bouncing ball during his ATP Qatar Open tennis tournament match against Latvia's Ernests Gulbis in Doha January 7, 2010.

"Last month, I had a shoulder problem, but I believe I'm back to my best. I may not serve as fast as Roger or Robin (Soderling), but if I can serve between 190 and 200 kph, that'll be fine."

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