Serena Williams shared a classy congratulations with U.S. Open winner Flavia Pennetta

In an all-Italian U.S. Open final, No. 26 Flavia Pennetta defeated No. 43 Roberta Vinci to earn her first-ever Grand Slam title Saturday.

After the 7-6 (4), 6-2 contest, Pennetta, 33, immediately announced her retirement.

"This was the perfect moment, I think," she said. "Was a really hard decision to make, but I’m really happy that I did it."

Vinci, a longtime friend of Pennetta's, claimed a spot in the final after stunning Serena Williams in the semifinals, ending the top-ranked player's bid to complete a calendar-year Grand Slam.

Williams congratulated Pennetta via Twitter.