Flavia Pennetta won her first grand slam singles title over Roberta Vinci in an improbable all-Italian U.S. Open final on Saturday then added one more shock to a stunning fortnight by announcing her retirement.

With the 7-6(4) 6-2 win, the 33-year-old Pennetta becomes the fourth oldest grand slam winner in the Open Era and joins 2010 French Open champion Francesca Schiavone as the only Italian women to win a major singles title.

But as the celebrations kicked into high gear, Pennetta dropped a bombshell that provided a dramatic finish to the year's final grand slam and her career.

After embracing childhood friend and Fed Cup team mate Vinci at the net a smiling Pennetta stood at center court during the trophy presentation and told a capacity crowd at Arthur Ashe Stadium that included Italian Prime Minister Matteo Renzi that she would retire.

"This is the way I would like to say goodbye to tennis," she said before hoisting the trophy and accepting the $3.3 million winner's check. "I'm really happy. It's what all the players seem to want to do, to go out with this big trophy.

"And so this one was my last match at the U.S. Open and I couldn't think to finish a better way."

Pennetta's surprise announcement provided a jaw-dropping finish to a grand slam packed with surprises.

She said it was a decision she made a month ago when a grand slam triumph would have seemed improbable and suggested fate may have played a part in her grand slam goodbye.

"Maybe that is why I am here today," Pennetta said while embracing the trophy. "I was trying to play every match like it was my last one. Trying to play best all the time.

"For me it is easy to practice and stay in this life but sometimes it is hard to compete.

"It will be a new life for me, I played tennis since I was young."

BREATHTAKING UPSETS

The unlikely final was set up by breathtaking upsets as unseeded Vinci knocked off world number one Serena Williams in the semi-finals to end the 33-year-old American's quest for a calendar year Grand Slam.

Pennetta's path to the final included two huge hurdles which she cleared with confidence, taking down Czech fifth seed Petra Kvitova in the quarter-finals and Romanian second seed Simona Halep in the semi-finals.

"Miracles can happen. Because I beat Serena, miracle," said Vinci. "And then two Italians can reach the Grand Slam final, a miracle."

While the all-Italian final was greeted by a big yawn in the Big Apple, Italy was gripped in tennis hysteria as the sport made front page headlines and convinced Renzi to drop his busy schedule and fly to New York.

Since Williams's triumph at Wimbledon in July, the buildup to the U.S. Open had focused almost entirely on the American's bid to become just the fourth woman and first since 1988 to complete the calendar slam.

Her surprise semi-final exit took much of the buzz out of the tournament and triggered an immediate collapse in ticket prices for the women's final.

According to ticket aggregator SeatGeek, the median price for tickets to the final on the secondary market had risen to more than $1,500 when Williams reached the semi-final but plunged below $500 following her loss on Friday.

The match between best friends and former doubles partners who first played each other when they were nine years old got off to a predictably cautious start for two players competing in their first grand slam final.

Showing signs of nerves, Pennetta and Vinci seemed content to battle from the baseline, trading early breaks as the first set went to tiebreak.

After winning the tiebreak and sensing the title was within her grasp, Pennetta broke Vinci at the first opportunity en route to 4-0 lead before clinching the match with a final service break.

"We spent so much time together, we moved to Rome together when we were 13-, 14-years-old and stayed in the same room for four years," said Pennetta. "It is like a sister, it is so magical you have one of your best friends with you in this moment."