By Julian Linden

NEW YORK (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.

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.

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.

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."

(Editing by Frank Pingue)