World No. 1 Serena Williams pulled out of the season-ending WTA Finals Singapore yesterday, putting a dampener on a season that had started off so brightly.

In a statement, the 21-time Grand Slam winner said she is "taking a proactive step and withdrawing from tournaments in Beijing and Singapore to properly address my health and take the time to heal".

The 34-year-old American has not played since Sept 11.

Then, she lost in the semi-finals of the US Open to unseeded Italian Roberta Vinci in one of the sport's most shocking upsets.

More significantly, the defeat cost this year's Australian Open, Wimbledon and French Open champion a chance at a historic calendar-year Grand Slam.

Williams, who also withdrew from next week's China Open in Beijing, added: "I've played injured most of the year - whether it was my elbow, my knee, or in the final moments after a certain match in Flushing, my heart.

"I'm a fierce competitor and I want to compete as well as I can, for as long as I can.

"This is a very difficult decision but one ultimately made because of the love of the game."

Citing "health" as the reason for her withdrawal came as a surprise as her statement comes just a day after her coach Patrick Mouratoglou revealed that she was deeply disappointed to have missed out on a calendar-year Grand Slam by just two wins and may be better off with a short break.

Williams - who won the past three Finals - may, however, still make a stopover during the Oct 23-Nov 1 showpiece at the Singapore Indoor Stadium.

According to Women's Tennis Association rules, a qualified player who cannot compete at the Finals must attend the tournament for a minimum of two days.

In addition, she must participate in at least three hours of promotional activities on each of those days.

Flouting the rules results in a US$125,000 (S$178,000) fine and a possible suspension.

"Serena's health has to be her No. 1 priority. The risk of injury is a reality in the world of elite athletic performance," said WTA president Micky Lawler yesterday in a statement. "We will miss seeing Serena on court in Singapore and wish her a speedy recovery.

"We have a superb line-up of top players already qualified, and still in contention for qualification, at the BNP Paribas WTA Finals Singapore presented by SC Global so fans can expect more outstanding, world-class competition on the court again this year."

The event was held for the first time in the Republic last year, with the next four editions also slated to be hosted here.

After sweeping the year's first three Major titles, Williams is one title short of tying German Steffi Graf's career total of 22 Grand Slam championships and three behind Australian Margaret Court's all-time record of 24.

She is one of three players, along with Romanian world No. 2 Simona Halep and Russian No. 3 Maria Sharapova, to have booked a spot in Singapore.

Sharapova faces a battle to get fit for the Finals, which features the world's top eight women's singles and doubles players. The Russian was forced to pull out of her China Open title defence with a left forearm injury but is upbeat about recovering in time for the Finals.

Spain's Garbine Muguruza, 11th on the Road to Singapore leaderboard, boosted her Finals hopes by beating Slovak Anna Schmiedlova 6-2, 6-2 in the quarter-finals of the Wuhan Open yesterday.

Czech Karolina Pliskova, 10th in the race to Singapore, fell 6-7 (2-7), 3-6 to the in-form Vinci.