After winning her first Grand Slam title at age 33, while seeded 26th, by beating the woman who beat Serena Williams, Flavia Pennetta provided this U.S. Open full of surprises with one last twist.

After meeting her opponent, Roberta Vinci, at the net for a long hug — both from the southern heel of Italy's boot, they were childhood friends, then doubles partners and roommates as teens — Pennetta let Vinci in on a little secret that would soon be shared with the world.

Pennetta decided about a month ago she would be retiring after this season, and this would be her last appearance at Flushing Meadows.

Talk about going out on top.

In one of the unlikeliest major finals in women's tennis history, Pennetta beat Vinci 7-6 (4), 6-2 in the U.S. Open final at Flushing Meadows on Saturday, then revealed during the trophy ceremony that she is ready to hang up her racket.

"This is how I say goodbye to tennis," Pennetta said as her fiance, tennis player Fabio Fognini, captured the scene with his phone's camera. "I couldn't think to finish in a better way."

Later, Pennetta clarified that she plans to enter two tournaments in China over the next month, and the season-ending WTA Finals in Singapore, if she qualifies. But she definitely won't be back at the U.S. Open.

"Why? Because sometimes it's getting hard for me to compete. ... If you don't fight every week in the same way I did today, it's going to be, like bad. ... And I don't have ... this power anymore, sometimes," Pennetta explained. "I mean, with this — winning today — my life is perfect."

She is the oldest woman in the Open era, which began in 1968, to become a Grand Slam champion for the first time. Vinci, who is 32, would have earned that distinction had she been able to follow her stunning upset of Williams in Friday's semifinals with another victory.

This was the first major final for either participant, and the first time since WTA computer rankings were instituted in 1975 that both U.S. Open women's finalists were ranked outside the top 20 (Vinci is 43rd). Pennetta entered the tournament with only a 17-15 record this season. Vinci was 20-20 in 2015, and 40-43 in majors for her career.

They grew up 40 miles apart in coastal towns in Puglia, a region on the heel of Italy's boot-shaped peninsula, and have been facing each other on court for two decades — with the stakes much lower, of course. They shared laughs and tears in the locker room Friday while watching a video of a TV interview they did back in 1999, when they won the French Open junior doubles title as teenagers.

Shocking as the timing of Pennetta's announcement was, the biggest shock of these two weeks — and, indeed, it ranks up there in the history of tennis — was Vinci's win against No. 1 Williams in the semifinals Friday. That stopped Williams' 33-match winning streak in majors and her attempt to become the first player since Steffi Graf in 1988 to win all four Grand Slam tournaments in a single season.

Vinci said the mental and physical exhaustion from that momentous triumph affected her Saturday.

"This morning, I woke up and thought, 'What did I do yesterday?!' I swear to God," Vinci said, resting her cheek on her left hand. "Poor thing, (Williams) fought for the Grand Slam, and I stole her dream. When I went to bed last night, I thought: 'I wonder how Serena feels.' ... Because she deserved it. I don't mean that match — she deserved to win all four Slams. She won three and was so close to (the fourth)."

Vinci charmed the crowd after beating Williams and again after losing to Pennetta, saying she wanted the champion's trophy, not the one for the runner-up, then pretending to steal the $3.3 million check.

"We know each other since forever," Pennetta said. "We spend so much time together, we could write a book about our lives."

Quite a chapter Saturday would provide.

Rain fell in the second set, then turned into a downpour after the match. The players sought shelter in the tournament referee's office, where they posed for pictures with Italian Premier Matteo Renzi, who said he was "proud of their determination and their tenacity."

The Arthur Ashe Stadium crowd — some folks had paid top dollar for tickets, in anticipation of seeing Williams take aim at history — was rather quiet, especially in the opening set. Perhaps it was difficult to decide which relatively unknown woman to cheer for.

Now, though, Pennetta will always be known as a Grand Slam champion, no matter how many more matches she plays.