Flavia Pennetta posing for photos after being presented with the US Open trophy.

Italy’s Flavia Pennetta became a shock grand slam champion by winning the US Open title and then revealed another surprise by announcing her retirement from tennis.

Pennetta had been rated 150/1 to triumph at Flushing Meadows before the tournament began but became the oldest female in the Open era to win a first major title after beating fellow Italian Roberta Vinci 7-6 6-2 late Saturday.

Vinci’s challenge was even more of a surprise, having knocked out top seed Serena Williams in the last four on Friday, but the world number 43 was unable to repeat her heroics in what was the Open era’s first all-Italian grand slam final.

Pennetta, aged 33, flung her racket into the air after sealing victory in one hour and 33 minutes, before embracing her compatriot with an affectionate hug at the net.

The champion addressed a jubilant crowd at a packed Arthur Ashe Stadium but as the interview concluded, she asked to take the microphone for one more announcement.

“I am going to say goodbye to tennis,” world number 26 Pennetta said.

“This is what all players want to do, go out taking a big trophy home. This is my last match at the US Open and I couldn’t finish in a better way.”

It was a suitably unpredictable finish to a baffling tournament that had just witnessed a final contested between two players with a combined ranking of 69, age of 65, and who had never before made a grand slam final between them.

It is only the seventh time a female player from outside the top 25 has won a major title and Pennetta’s success in her 49th grand slam is the longest a woman has ever had to wait to achieve the feat.

“I am really happy, I have to say before this tournament I never think to be so far, I never think to be champion,” Pennetta said.

“That’s why it’s amazing, this is coming as a big surprise for me. It’s a dream come true.”

Only two weeks ago, Pennetta and Vinci, girls doubles winners together at the 1999 French Open, were having dinner in New York, oblivious to the extraordinary tournament about to unfold and which will surely define both their careers.

Vinci admitted her shock win over Williams, which ended the American’s hopes of a first calendar grand slam since 1988, had been the greatest day of her life and perhaps it was too much to expect the 32-year-old to recover her poise less than 24 hours later.

“It shows miracles can happen,” Vinci said.

“I beat Serena – a miracle. Two Italians reach the grand slam final – a miracle. One Italian wins a grand slam – a miracle.”

Pennetta’s run to the final was less mind-boggling but no less impressive, given she had to beat Samantha Stosur, Petra Kvitova and Simona Halep along the way.

She plans to play in Wuhan, Beijing and, if she qualifies, the WTA Finals in Singapore before hanging up her racket at the end of the year.