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Teammates one afternoon, opponents the next, Sara Errani and Roberta Vinci exchanged glances and giggles while sitting side-by-side for a news conference after reaching the U.S. Open doubles semifinals with a victory on Tuesday.

When a reporter asked about the Italians' singles quarterfinal against each other, scheduled for Wednesday, Errani interrupted.

"Hey, it's a news conference with both of us here," she protested kiddingly, then added in a sarcastic voice: "Go ahead, ask her how she'll play tomorrow."

Vinci then pretended to start running through her strategy — "topspin backhands; I'll stand 7 meters behind the baseline" — and Errani interrupted again: "Come on, you can't ask us questions about that!"

Their 6-2, 7-6 (2) victory over the 11th-seeded pair of Germans Julia Goerges and Kveta Peschke assured Errani of moving up to No. 1 in the WTA doubles rankings, with Vinci at No. 2. Errani and Vinci won the French Open doubles title in June.

The American duo of Liezel Huber and Lisa Raymond had been co-No. 1s, but they lost in the third round at Flushing Meadows.

In singles, the 10th-seeded Errani or 20th-seeded Vinci will give Italy its first female semifinalist at the U.S. Open since the professional era began in 1968.

Errani will be playing in her third career singles Grand Slam quarterfinal — she was the runner-up at Roland Garros this year — while Vinci makes her debut at that stage in her 32nd major tournament.

"Playing a Slam quarterfinal for the first time is amazing. There definitely will be some jitters, but I think that's normal," Vinci said. "She's may be used to this sort of thing."

Right on cue, Errani corrected her doubles partner and pal.

"I wouldn't say I'm used to it," Errani objected with a smile. "Not quite."

Over and over, they looked at each other before answering. They chuckled at each other's punch lines, rocking in their black leather chairs.

And they assured everyone the plan would be the same Tuesday night as it is every evening during a tournament they're both entered in.

Dinner — together.

Back to the same hotel — together.

They've even been wearing the same outfits on court, white in doubles and pink in singles. Errani said that wouldn't change Wednesday, either.

"A simple day, just like every other," Vinci insisted.

This will be their sixth match as foes in singles, and Errani leads 3-2.

"There will be tension, the way there always is in any match, the way there was in the first round, the second round, the third round. It's a quarterfinal," Errani said. "Maybe, in theory, we'll actually be a little calmer, because we know one of us moves on in the tournament. One of us will win, one will lose."

Errani and Vinci aren't the only women's doubles team that wound up getting sent to opposite sides of the net in singles at Flushing Meadows this year. Vania King of the U.S. and Yaroslava Shvedova of Kazakhstan, the 2010 Wimbledon and U.S. Open doubles champions, met in the first round; Shvedova won in straight sets, and both nearly cried afterward.

"Our friendship is more valuable than a tennis match, even if it is a quarterfinal or semifinal at a Slam," Errani said.

Then, using a mock-serious voice, she added: "But tomorrow, I hope to not lose a friend."

And with that, Errani and Vinci both dissolved into laughter.

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Follow Howard Fendrich on Twitter at http://twitter.com/HowardFendrich