World number two Maria Sharapova said on Saturday she was not surprised to see players withdrawing from the end-of-year "Asian swing" tournaments after Serena Williams pulled out of the China Open.
The 25-year-old Russian, whose victory at Roland Garros in June completed her career Grand Slam, said the "long" WTA season meant that by the time Beijing came along energy levels were often down.
"This is the end of the year for us and you know it's a pretty long season of 10 months of playing tournaments back to back," she said.
"There is no doubt towards the end of the season you are not as fit as you are in the beginning, maybe don't have as much energy, because the last Grand Slam of the year (US Open) was only a couple of weeks ago.
"You try to give your all for that. And then also we had the addition of the Olympics this year. So it was an incredibly busy summer for everybody."
Williams, winner of the US Open and Gold at London -- plus the Wimbledon title -- in what has been a fantastic year for the American, withdrew from the China Open on Thursday because of flu, just two days before the start.
It was the second year running that the former world number one, who has 15 Grand Slam singles titles to her name, has not competed in Beijing, which is the fourth and final compulsory tournament in the women's calendar.
Asked if she had any thoughts on Williams' withdrawal, Sharapova said: "It's a long year. You're not really surprised to see a few more withdrawals than at the beginning of the season."
September sees the tour come to Asia and is a particularly hectic month with consecutive events in Seoul, Guangzhou, Tokyo and Beijing falling just weeks before the end-of-season $4.9 million WTA Championships in Istanbul.
Earlier this week, world number one Victoria Azarenka, the losing finallist in this year's US Open, pulled out of the Pan Pacific Open in Japan at the quarter-finals citing fatigue.
Sharapova, the second seed, starts her bid for the China Open with a first-round tie against unseeded Simona Halep of Romania.