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FLUSHING MEADOWS, N.Y. -- Agnieszka Radwanska looked little like the player she was earlier in the tournament. As the No. 2 seed in the U.S. Open, the fourth-round match against No. 20 Roberta Vinci should have been business as usual on Labor Day.

[+] EnlargeDon Emmert/Getty ImagesAgnieszka Radwanska of Poland found the going tough in her upset loss to Italy's Roberta Vinci.

But Radwanska could not hold serve, she moved poorly and, early in the 6-1, 6-4 loss, it became apparent she would not hold the day -- even though Radwanska held a 4-0 record against the Italian.

"It's always disappointing, especially when you're losing in a Grand Slam," Radwanska said. "I think I'll have to leave it at that right now."

This has been a good year for Radwanska, but it won't be the year she endears herself to the American audience. Instead, there will be an all-Italian quarterfinal when the 29-year-old Vinci plays countrywoman and doubles partner No. 10 Sara Errani, who upset No. 6 Angelique Kerber earlier Monday.

"I'm happy to play against her," Vinci said. "For sure, an Italian goes to the semifinal. She's my best friend, so I'm very happy to play against her."

Meanwhile, Radwanska joins one of her best friends, Caroline Wozniaki, who also exited the tournament early.

Vinci had 13 break points in the first set alone and needed only five of them. Radwanska put up more of a fight in the second set, but it was too little and late. She did not cite an injury, though her shoulder was taped as it usually is these days, but instead talked about fatigue from the long year on the WTA Tour.

"It's always difficult, especially the U.S. Open, the last of the Grand Slams," Radwanska said. "It's always tough because you know, the season is very long. I really played a lot this year. Sometimes it's really hard to stay focused all the time because you're really tired after the whole season."

And this isn't the end of her season. The WTA Tour rolls on to Asia once the Open is over. There is very little time to catch your breath after a disappointing result in a Grand Slam a continent removed from where you live and a continent away from where you are going.

Radwanska caught just enough of Serena Williams' match to say that Williams is playing so well, she would have to be considered the favorite to win.

Radwanska isn't that dominant yet. She is working toward it and has many of the physical tools to get there. It would be a mistake to think a disappointing Open would mean she won't soon contend for a Grand Slam championship.

"It was the best season of my career, first major final, first time No. 2 in the world -- I can't complain about that, for sure," Radwanska said. "Of course, it's not over. I would like to defend some of my points from Asia and of course play my best tennis for the [WTA Championships]."