On the day she was welcomed into the exclusive club of tennis' Grand Slam champions, Flavia Pennetta listened to her body and dropped the bombshell.

All her life, she had trained for that moment of glory. But when it came, when she was hoisting the US Open trophy - her first major title - aloft at Flushing Meadows, she knew it was time to retire. And she announced in her finest hour that she would bow out at year-end.

Hence, the Italian was naturally sympathetic when she learnt that world No. 1 Serena Williams had withdrawn from the Oct 23-Nov 1 WTA Finals in Singapore, citing a need to protect her health.

After all, she knows what it means to have no fuel left in the tank, no ammunition in the arsenal.

Pennetta told The Straits Times yesterday in a telephone interview courtesy of the Women's Tennis Association: "I didn't hear about this (Williams' withdrawal) until this morning. It's her decision.

"She knows how she feels.

"We must trust her that she knows that this is the right time for her to recharge her batteries.

"We have to understand that."

Last month, Pennetta, then the 26th seed and 10,000-1 outsider in the tournament, staged a fairy-tale run to win the US Open.

She claimed a string of scalps, starting from world No. 4 Petra Kvitova in the quarter-finals before vanquishing No. 2 Simona Halep in the semi-finals.

She beat Roberta Vinci 7-6 (7-4), 6-2 to win the all-Italian final.

"That is how I would like to say goodbye to tennis," she said during the trophy ceremony.

At 33, she was the oldest woman to win a maiden Grand Slam.

Never ranked higher than 12th, she has now shot up to sixth and on course to qualify for the WTA Finals, the prestigious season-ending gathering of the top eight singles players and doubles teams.

"No, I am not reconsidering," she insisted when asked whether she would relook her plan to retire.

"I made that decision because that was what I was feeling. That moment, I knew I wanted to stop."

Singapore will get to see her swansong at the WTA Finals if she stays in the upper reaches of the rankings. "Almost there, almost there," she said with laughter on her hopes of earning her ticket.

"This season, I didn't play so well. But I've always liked the US Open and the city. I felt it will be a good way to go. So I just went match by match and paid no attention to the draw.

"The WTA Finals will be difficult. But it'll be really nice if I can qualify and finish at a really big tournament."