Serena Williams may be set for an extended break to finish the year, according to her coach Patrick Mouratoglou.

The American world number one last played at the US Open, where her bid for a calendar Grand Slam came to an end in the semi-finals.

Williams, 34, may remain sidelined to finish 2015, but Mouratoglou said it all came down to her motivation levels.

"I don't doubt she will have the motivation to win more grand slams and reach records," he told ESPN on Tuesday.

"I'm just thinking about the end of the season, and I don't know how high the motivation is, and I don't think she should go play competitions if her motivation is not high enough."

Williams has enjoyed an incredible year, winning three grand slam titles and holding a 53-3 win-loss record.

Mouratoglou said the loss at Flushing Meadows had a huge impact on Williams.

"She was two matches away from something really big, so it was very painful," he said.

"Any loss is very painful for her, but this one even more than usual, so it just takes time to recover from it.

"When the motivation comes back, which I don't doubt it will, then it will be time to start tournaments again."