Venus Williams on Friday reached her first semi-final of 2012 as she struggled past Samantha Stosur 6-2, 6-7 (2-7), 6-4.

CINCINNATI, Ohio – Venus Williams on Friday reached her first semifinal of 2012 as she struggled past Samantha Stosur 6-2, 6-7 (2-7), 6-4, leaving the US Open champion scratching for form heading into her title defence.

The 32-year-old Williams battled in a third set, after failing to serve out the win at 5-2. But she nailed the victory in her second try at 5-4, ending in two and a half hours with six aces.

The American has won six of seven career matches with Stosur, who has a negative 4-5 record since Roland Garros, where she reached the semi-finals.

But there was little joy for Williams' sister Serena, whose 19-match win streak was ended by German fifth seed Angelique Kerber 6-4, 6-4.

Williams smashed a racquet in anger as she went down to her first loss since the French Open in May. Kerber double-faulted on her first match point but aced on her third to ice the win and reach the semi-finals

Venus, a seven-time Grand Slam winner, was delighted to get through after her average season as she lives with an energy-sapping syndrome.

“I was not nervous when I served for the match,” she said. “But Sam is tough, she puts so much spin on the ball and plays so aggressive. I was able to execute my shots in the end.

“I needed to work the points in the last game. I'm happy to get a win over a top 10 player. It's great to have this kind of a win you know you will be playing people on their games.”

On the men's side, Toronto champion Novak Djokovic ran his record over Croatian Marin Cilic to 7-0 with his 6-3, 6-2 defeat for a semi-final spot.

The victory set up a re-match of an Olympic disappointment as he faces Argentine Juan Del Potro after losing to the South American a fortnight ago in the London Games bronze medal match.

Del Potro moved through in Cincinnati over Frenchman Jeremy Chardy 6-1, 6-3.

Del Potro is trying to ignore problems with his left wrist but will have it checked out next week at the Mayo clinic in Minnesota before playing the US Open from August 27.

“I'm very confident with my doctor. He's the best of the world about the wrist, so after here I will talk with him and we will see. The good thing, I'm not getting worse so I can play.”

Djokovic was pleased with the showing which put him into the final four as he prepares to defend his US Open title.

“The match with Del Potro won't be for revenge, This is sport, you win, and you lose. He is playing with confidence and it will be a big challenge to play him.” – Sapa-dpa