This was certainly not in the script. It was supposed to be as easy as one and two, after Serena Williams beat her big sister Venus in the 2015 US Open quarterfinals.

How difficult was it really going to be for Serena to beat the unseeded Roberta Vinci in the semifinal, and then do the same to either Simona Halep or Flavia Pennetta in the finale to complete the Calendar Grand Slam? Easy enough, everyone thought.

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But, nope, Vinci had another script in mind and she tore the printed one to shreds to tear apart Serena's dreams of being the first tennis player since Steffi Graf in 1988 to win a calendar Grand Slam.

A place in the final looked like being a foregone conclusion when Serena cruised in the first set, winning it 6-2, but then suddenly she tightened up, and Vinci, sensing an opening, decided to play the best match of her life.

Serena crumbled and tumbled and Vinci, only a year younger than Serena at 32, soared farther than she has ever before in a Grand Slam to move into the final with a 2-6, 6-4, 6-4 victory.

"I didn't realize now yet, but it's amazing," Vinci said, admitting the incredible win hadn't sunk in yet. "It's like a dream. I'm really happy, but of course I'm a little bit really sad for Serena because she's incredible player, No. 1, almost career -- all the Grand Slams.

"But what I have to say? I'm happy. I don't know. It's tough to explain my emotion right now. Maybe tomorrow morning [Saturday] I can tell you something, but now it's amazing. It's magic moment for me. You work so hard for a long time, and it's incredible."

Serena seemed a little lost for words, talking to reporters after the match, with that tetchiness coming to the fore.

Asked how disappointed she was to lose the match, Serena gave a curt reply: "I don't want to talk about how disappointing it is for me," the American world number one said. "If you have any other questions, I'm open for that."

Serena was a lot more forthcoming about how good Vinci played, though.

"I thought she played the best tennis in her career," she added. "You know, she's 33 and, you know, she's going for it at a late age. So that's good for her to keep going for it and playing so well.

"Actually, I guess it's inspiring. But, yeah, I think she played literally out of her mind."

Awaiting Vinci now in an all-Italian final is Pennetta, who was just too good for the No.2 seed Simona Halep, with the No.28 seed winning 6-1, 6-3.