Serena Williams of the U.S. returns to Anastasia Pavlyuchenkova of Russia at the U.S. Open tennis tournament in New York
Serena Williams of the U.S. returns to Anastasia Pavlyuchenkova of Russia at the U.S. Open tennis tournament in New York, September 8, 2011. Reuters

The U.S. Open semi-final between Serena Williams and Caroline Wozniacki on Saturday is a mouth-watering clash of former and current world number ones and a chance to restore the natural order of tennis.

Wozniacki is the current world number one but the jury is out on whether she really is the best player in the world as she has yet to win a grand slam title.

The 21-year-old Dane has been pestered about her right to the number one status for most of the past year and bristles at the suggestion she does not deserve the ranking.

I don't care what people think and say or do, Wozniacki said. I'm number one in the world at the moment and I've been playing well and I have had a great year.

Wozniacki has dropped one set all tournament and now gets a chance for vindication but the stakes could not be higher.

If she can beat Williams then win the final, the arguments will cease. But if she loses, her status as the world number one will come under even more scrutiny.

Williams is unchallenged as the greatest player of her generation and one of the finest to ever grace the game. The American has 13 grand slam titles, including three U.S. Opens, but her ranking has tumbled to 29 given an 11-month layoff because of health problems.

She has won each of her matches at Flushing Meadows in straight sets so far but still has a point to prove after leaving her last U.S. Open appearance two years ago with a foul-mouthed tirade against a lineswoman.

Williams was fined and given a two-year probation but the 29-year-old has been on her best behavior this year and had nothing but praise for Wozniacki.

She never gives up, Williams said about Wozniacki. That's probably the biggest weapon I think you can have in all of tennis.

Williams has won her two previous meetings with Wozniacki but they offer few real guidelines as she needed a third-set tiebreak in Sydney in 2009 while the Dane retired hurt from their meeting at the Tour Championships two years ago.

Saturday's winner will almost certainly enter Sunday's final as the clear favorite since the bottom half of the draw produced two surprising semi-finalists in Australia's Sam Stosur and unseeded German Angelique Kerber.

Neither has won a grand slam singles title although Stosur made the French Open final last year and is one of the few players in the women's game with the power to match Williams.

Stosur had a rollercoaster ride to the last four. She was involved in the longest women's match ever played at the U.S. Open when she beat Russia's Nadia Petrova in the third round, lasting more than three and a quarter hours.

In her fourth round clash with Russian Maria Kirilenko, she played the longest tiebreaker in a women's match at any grand slam. She lost the 32-point tiebreaker but won the match.

Stosur then offered a glimpse of what she is capable of when her game is right when she crushed world number two and last year's runner-up Vera Zvonareva 6-3 6-3 in 67 minutes in the quarters.

Kerber, ranked 92nd in the world, has slipped through the draw almost unnoticed. She had never been past the third round of a grand slam before this week and has never won a WTA Tour event but beat two seeds, including Italy's Flavia Pennetta, to get to the semis.

The pair have never played each other before and Stosur looms as the strong favorite but the Australian remains wary.

It's the semis of a slam, and we all want to get through to that final, the Australian said. I'm going to have to play well. It's definitely going to be a good challenge.