UPDATE 2-Tennis-Open-Serena labels opponent "a cheat"

* Serena accuses opponent of cheating

* Advances to fourth round with three-set win

(adds more quotes)

By Julien Pretot

PARIS, May 30 (Reuters) - Serena Williams accused her Spanish opponent of cheating on a controversial point in a 4-6 6-3 6-4 third round defeat of Maria Jose Martinez Sanchez at the French Open on Saturday.

Martinez Sanchez bagged an ill-tempered opening set after breaking in the fifth game with a controversial shot which appeared to have bounced off her arm.

"I didn't think the ball touched her. The ball did touch her 100 percent on her arm," the American second seed, who will next face Canada's Aleksandra Wozniak, told a news conference.

"The rules of tennis are when the ball hits your body, then it's out of play. You lose a point automatically. So the ball hit her body, and therefore, she should have lost the point instead of cheating.

"I hit that ball rather hard. She knew that ball hit her," she added.