NEW YORK (AFP) - With a historic calendar Grand Slam attempt shattered before the weekend, a US Open women's final missing much of its anticipated marquee magic now faces the threat of thunderstorms.

Italy's Roberta Vinci and Flavia Pennetta are set for a Saturday afternoon trophy showdown at Arthur Ashe Stadium in its final weekend of matches before a retractable roof is ready for the 2016 tournament.

"One Italian will win for sure," Vinci said. "It's an incredible moment."

She defeated defending champion Serena Williams 2-6, 6-4, 6-4 while Pennetta ousted Romanian world number two Simona Halep 6-1, 6-3.

Williams was bidding for the first calendar Grand Slam since Steffi Graf in 1988 and a 22nd Slam singles crown to match Graf's Open Era record.

But all that vanished when Vinci pulled one of the greatest stunners in tennis history.

"I don't want to talk about how disappointing it is for me," Williams said.

And now a forecast calls for a 65 per cent chance of thundershowers on Saturday and a 75 per cent chance for the same on Sunday for the men's final between Novak Djokovic and Roger Federer.

The only Italian woman to win a Grand Slam singles title was Francesca Schiavone at the 2010 French Open. The only other woman to reach a Slam final was Sara Errani, who lost the 2012 Roland Garros final.