It is the most improbable final in women’s grand slam history, unfancied Italian thirty-somethings Flavia Pennetta and Serena slayer Roberta Vinci squaring off for US Open glory.

Neither veteran – both former world No.1 doubles exponents – considered themselves a hope of making the singles final, let alone winning it.

Certainly not Vinci, who’d booked a flight out of New York on Saturday, so little chance did she give herself of ending Serena Williams’ seemingly inevitable pursuit of the fabled calendar-year grand slam.

“I called my travel agency to say, `Okay, book me a flight because you know’ … and then now I have my final tomorrow,” Vinci beamed after stunning the top-seeded three-time defending champion 2-6 6-4 6-4 in one of the sport’s greatest upsets.

“It’s incredible. This is the best match I played in my life.

“It’s a magic moment for me. You work so hard for a long time. It’s amazing. It’s like a dream.”

A first-time grand slam semi-finalist, the veteran world No.43 had lost all four previous clashes with Williams in straight sets and stared down the barrel of another mauling after the American raced through the opening set on Friday.

This time, though, the tennis gods were smiling as Vinci raised her game, as Williams lost her’s with it all on the line, to pull off the shock of the year and deny the world No.1 a special place in the sporting record books.

“It’s incredible. I’m 32, almost at the end of my career and then I make the first US Open final in a grand slam. I didn’t expect this,” Vinci said.

“I’m really happy but of course I’m a little bit really sad for Serena because she’s an incredible player – almost to win all the grand slams. She deserved to win. She’s the No.1.”

Steffi Graf last achieved the grand slam sweep in 1988.

Williams’ bid to catch Graf’s open-era benchmark of 22 grand slam singles titles and draw within two of Australian Margaret Smith Court’s all-time record is also on hold.

“I don’t want to talk about how disappointing it is for me,” Williams said after having her 33-match grand slam winning streak spectacularly snapped.

Vinci and Pennetta – Fed Cup teammates and former doubles partners – will make history instead in the first-ever all-Italian grand slam singles final.

Pennetta, who heads Vinci 5-4 in head-to-head meetings, continued her own giant killing run to also reach her maiden major final.

The 26th seed routed world No.2 Simona Halep 6-1 6-3 in less than an hour, three weeks after admitting she’d thought her time had passed.

A fourth-round winner over Australia’s 2011 champion Samantha Stosur, who’d been the last conqueror of Williams in New York, Pennetta even almost quit the game last year while battling a depressing wrist injury.

“I was close, really close,” the 33-year-old said.

“But it’s easy for me doing this kind of life because it’s nice life.

Apart from taking out Stosur and now Halep, Pennetta also upset two-time Wimbledon winner and fifth seed Petra Kvitova en route to the championship match.

“I didn’t expect to make it this far,” she said.

“I just try to play my best every match. I’m taking it match by match.

“I’m playing very well and it’s amazing to be here.”