Roberta Vinci knows what to expect Thursday night when she faces Serena Williams in a women's semifinal at the U.S. Open -- a barrage of big serves and a fusillade of heavy groundstrokes.

The 32-year-old Italian has a wealth of experience. She also knows that no matter that wealth, she might not be rich enough in expertise to overcome the No. 1 player in the world.

"I know I have a lot of experience, but when you play against Serena, doesn't matter," Vinci said after she won her quarterfinal over Kristina Mladenovic on Tuesday. "You have to play better, then better, then better."

It was nearing midnight on Tuesday and Williams, already late to her mandatory media conference, was trying to find the words and the energy to answer questions, the inevitable questions that come when she faces her sister Venus.

Serena had expended a whole day's worth of energy to beat Venus in three sets in a quarterfinal in Arthur Ashe Stadium before a ramped-up crowd. The whole night suggested championship final, though it was only the fifth round. That would mean two more rounds to go for Serena to achieve the calendar Grand Slam and win her fourth straight Open title and tie Steffi Graf for second all-time with 22 total Slams. Margaret Court leads with 24

Up next for her in the semifinals is the very unlikely Vinci. Venus figured to be Serena's toughest opponent on the way to the semis, and she was. The level of play was high, the competitive level extreme.

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In Vinci, Williams faces someone who hasn't beaten her in four matches, hasn't taken a set off her and never won more than four games in a set. Vinci, once ranked No. 11 in the world, is now No. 43. It didn't seem likely at all she would advance as far as even the quarters, but seeds that she would have faced in earlier rounds were upset and her fourth-round opponent, Eugenie Bouchard, had to withdraw with a concussion. That meant she didn't beat a single seeded player to reach the semifinals, the farthest she's ever been in a Slam.

She would seem an easy mark for Williams. But in deference to the occasion and her opponent, Williams wasn't going to say it.

She referenced beating Vinci in the last meeting in Toronto in August, a two-set win. Vinci plays with guile, can play at the net -- she's won five Slam doubles titles with Sara Errani -- and generally tries to hang in and hope for the best.

"She played me really tough, and I didn't really expect that," Williams said. "That's how I sprained my finger, actually, was playing against her . . . She's not in the semifinals of a Grand Slam for no reason. She knows what do and she knows what to play. I just think it was great that I played her because I kind of know what to expect, and I'll be ready for it this time."

"She's an incredible player," Vinci said. "The serve, it's incredible."

@Newsday

One way or another, Vinci is in for an incredible experience tonight.