CINCINNATI - World No. 1 Roger Federer moved closer to a fifth ATP and WTA Cincinnati Masters crown on Friday as the Swiss star crushed American Mardy Fish 6-3, 7-6 (7/4) to reach the semi-finals.

Federer, seeking his sixth title of the year, was in regal form as he beat Fish for the eighth time in nine meetings. The American has lost 17 of 20 sets in the one-sided rivalry. Federer will line up  against compatriot Stanislas Wawrinka after the Swiss stormed back to beat Canadian Milos Raonic 2-6, 7-6 (7/5), 6-4, in another quarter-final.

In women's play, Angelique Kerber ended the 19-match win streak of Wimbledon and Olympic champion Serena Williams with a 6-4, 6-4 victory.

The German fifth seed double-faulted on one match point but concluded with a winning ace on another to send the second seed out of the event, one that Williams has never won.

China's Li Na reached the semi-finals over top seed Agnieszka Radwanska 6-1, 6-1 as the Polish favourite struggled with a shoulder injury.

Federer was playing a Cincinnati quarter-final for the sixth time in seven years as he aims for a 76th career trophy. He owns Wawrinka 11-1 overall in their personal rivalry.

"I played really well," Federer said. "The second set was very competitive, but I was able to maintain a high level for a long time. I'm very pleased with the form and glad to reach the semis." US Open champion Novak Djokovic booked a semi-final with Olympic bronze medallist Juan Martin del Potro by beating Marin Cilic. It will be a rematch of the bronze medal match at London.

"The match with Del Potro won't be for revenge," Djokovic said. "This is sport. You win and you lose. He is playing with confidence and it will be a big challenge to play him."

Serb second seed Djokovic took only 80 minutes to dispatch the Croatian 6-3 6-2, improving to 7-0 in their personal series.

"It was a very good match, my best of the tournament," Djokovic said. "I'm on good form. I may have grown up on clay, but I've achieved my best career success on hardcourts."

Argentine Del Potro advanced by ending the giant-killing run of Frenchman Jeremy Chardy 6-1 6-3. Del Potro has won his past two matches against Djokovic, including the bronze showdown and in a Davis Cup tie last September.

Del Potro will travel to the Mayo Clinic next week to have his sore left wrist checked out by the same doctors who operated on his right wrist in 2010. Williams destroyed a racquet in a fit of anger during the loss but will now have the rest days she has been seeking during her summer of success ahead of the start of the US Open, which begins on August 27.  "I was nervous at the end but I told myself to just keep hitting the ball," Kerber said. "Serena has been playing well. This was a good win for me and I'm very pleased."

It was the opposite story for elder Williams sister Venus, who upset defending US Open champion Samantha Stosur of Australia 6-2, 6-7 (2/7), 6-4.

The 32-year-old American twice had to serve for victory in the final set, nailing it on her second attempt from 5-4 after 2 1/2 hours. "It's always disappointing to lose when you play a good match like that and you think you've done enough almost to win," Stosur said. "Still, I had three really good matches here and can get confidence and gain a bit of momentum going into New York."

Women qf results:

Venus Williams (USA) bt Samantha Stosur (AUS x3) 6-2, 6-7 (2/7), 6-4. Angelique Kerber (GER x5) bt Serena Williams (USA x2) 6-4, 6-4. Li Na (CHN x9) bt Agnieszka Radwanska (POL x1) 6-1, 6-1. Petra Kvitova (CZE x4) bt Anastasia Pavlyuchenkova (RUS) 6-3, 7-6 (7/4.)

3rd round: 

Anastasia Pavlyuchenkova (RUS) bt Caroline Wozniacki (DEN x6) 6-4, 6-4. Li Na (CHN x9) bt Johanna Larsson (SWE) 6-2, 6-2.

Men qf results:

Juan Martin Del Potro (ARG x6) bt Jeremy Chardy (FRA) 6-1, 6-3, Novak Djokovic (SRB x2) bt Marin Cilic (CRO x12) 6-3, 6-2, Stanislas Wawrinka (SUI) bt Milos Raonic (CAN) 2-6, 7-6 (7/5), 6-4, Roger Federer (SUI x1) bt Mardy Fish (USA x10) 6-3, 7-6 (7/4).