Novak Djokovic beats Blaz Kavcic at Serbia Open

Novak Djokovic
Djokovic has not lost since the ATP Finals in London last November

Novak Djokovic remains unbeaten in 2011 after easing past Blaz Kavcic to reach the semi-finals of the Serbian Open.

The home favourite won 6-3 6-2 to extend his winning run to 28 matches, 26 of which have come this year.

Djokovic will next face compatriot Janko Tipsarevic, who earlier beat Somdev Devvarman of India 6-3 3-6 6-4.

In the other half of the draw, Spaniard Feliciano Lopez beat Albert Montanes 6-4 6-4 and Italy's Filippo Volandri defeated Marcel Granollers 6-2 6-4.

Djokovic faced just one break point as he saw off Slovenia's Kavcic in one hour and 25 minutes on the clay, and so registered the best start to an ATP tour season since Ivan Lendl in 1986.

It was not a completely trouble-free win for Djokovic, who threatened to quit the match when leading 4-1 in the second set because he said the lines got too slippery as it started raining.

"I don't want to play, I don't want to get injured," he shouted at the umpire, leaving the court and sitting on his chair. The play resumed only after the lines were wiped, and Kavcic then managed his only break of the match.

In Portugal, Argentine Juan Martin del Potro beat top seed and world number five Robin Soderling 6-4 7-5 in the quarter-finals of the Estoril Open.

The 2009 US Open champion, who missed most of last year after wrist surgery, came through a rain break to defeat two-time French Open runner-up Soderling.

"It was very tough mentally today," said Del Potro. "You had to keep focus on court, in the locker room, in the player's lounge all during the delay. But I managed that well.

"I really wanted to win and I'm pleased with how I did. It was a good victory for me today."