Djokovic powers into semis

Beijing - Reigning China Open champions Novak Djokovic and Serena Williams experienced contrasting fortunes at the tournament on Friday, with the men's top seed powering through to the semi-finals, while Williams withdrew injured.

Djokovic, who has an incredible 100 percent record at the $5.4 million tournament, beat fifth seed Grigor Dimitrov 6-2, 6-4.

The world number one was again highly emotional on the Diamond Court at China's National Tennis Centre, smashing his racket twice in the second set in frustration.

The Serb broke his Bulgarian opponent's service twice in the first set, after saving three break points in the third game.

He appeared to be heading for a routine victory at 5-3 and serving for the match in the second set, but Dimitrov broke serve to temporarily halt Djokovic's advance.

But Djokovic took the next game and wrapped up his victory in 90 minutes.

Djokovic is bidding for his fifth title in Beijing, a tournament he has won every year since 2009 apart from 2011 when he did not take part.

His victory means he could face Briton Andy Murray in the semi-finals if the sixth seed in Beijing wins his quarter-final against Croatian Marin Cilic.

The duel would be a re-match of their epic US Open quarter-final which Djokovic won 7-6 (7/1), 6-7 (1/7), 6-2, 6-4.

Meanwhile in the women's competition, world number one Serena Williams withdrew from the tournament with a left knee injury.

The reigning champion in Beijing was due to play Australian Samantha Stosur after winning her third round match Thursday evening with a bandaged knee.

"It's really frustrating, especially from here. This is a really big tournament for me. I'm defending champ," the 33-year-old said.

Stosur, the 2011 US Open winner and former world number four, will now go straight into the semi-finals where she will play Petra Kvitova.

Third seed Kvitova beat Italian Roberta Vinci 7-6 (7/2), 6-4 to book her place in the last four.

The world number three has enjoyed a return to consistency in China after a poor run in the North American swing.

The Czech player won the inaugural Wuhan Open last Saturday, her third title of the year.

The $5.4 million China Open will reach its climax on October 5.