Serena and Stosur eye French Open final

he 34-year-old American would be the oldest ever women's champion if she captures the crown for the fourth time on Saturday.

But first she needs to see off the unexpected challenge of unseeded Dutchwoman Kiki Bertens.

Stosur, at 32 would contest her second French Open final after 2010 if she gets the better of rising Spanish player Garbine Muguruzu.

Williams almost saw her hopes of winning a record-equalling 22nd Grand Slam title in Paris blasted out of the water in a nervy 5-7, 6-4, 6-1 quarter-final win over lowly-ranked Yulia Putintseva.

Her post-match comments matched her perplexing performance.

"Yeah, I just was not playing my best. I kept missing, you know, just misfiring," she said.

"Honestly, at one point I didn't see the light at the end of the tunnel. I don't know. I just was -- I guess I was not the most positive mentally, but obviously I didn't want to stop."

Bertens, she said, would be another tough challenge despite her low ranking.

"Well, obviously she has a big serve and a big forehand. She really moves the ball around well," she said.

The 24-year-old Bertens knows she has nothing to lose going up against Williams, who she has played only once before, losing in straight sets in the second round of last year's US Open.

But she is fearful that the busy playing schedule she has kept over the last few weeks and the after-effects of her 7-5, 6-2 last eight win over Timea Bacsinszky could hamper her chances of pulling off what would be a huge shock.

"I have some problems with my calf," she said. "I'm just gonna prepare again for tomorrow, do everything what I can with the physios, and we will see how it is tomorrow."

The other semi-final features Australian experience and guile up against Spanish youth and power

- Unchartered territory -

Stosur is a veteran of 15 years on the WTA circuit who won the 2011 US Open, defeating Williams in the final, and finished runner-up at Roland Garros the previous year.

Her game revolves around a big, swinging serve, plenty of top spin, changes of rhythm and her years of experience.

Muguruzu is 10 years younger and her biggest achievement to date was reaching last year's Wimbledon final where she lost to Williams.

The Venezuelan-born Spaniard is in the modern mould of tall women players reliant on power-hitting from the baseline.

That throws up an intriguing clash of styles that draws fourth-seeded Muguruza into the unchartered territory of a Roland Garros semi-final.

"I don't think I have played against her. Maybe. Maybe. I know that she reached the final at the French Open," she said after her quarter-final win.

As it turns out they have played, just the once, on clay at Madrid in 2014 when Stosur won in three sets.

"She plays well. It will be a tough match. In a semi-final match, all matches are tough. Lots of players are under stress and they want to reach the final, so I will focus on my tennis and I will give my best," Muguruza said.