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.

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.

"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."

 

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

 

Spain's Rafael Nadal returns a shot against Potito Starace of Italy during the ATP Qatar Open tennis tournament in Doha January 6, 2010. [Xinhua/Reuters Photo]