"I don't know when Serena will retire, also because she doesn't know herself," Mouratoglou told Adnkronos in Rome during the Italian Open.
"But I am certain she'll decide to lay down her racket when she feels shes' no longer the best. She won't be like Roger Federer, who hasn't won a Grand Slam in four years, despite still playing at the highest level."
The 34-year-old Williams one sailed through to the quarter finals of the Italian Open in Rome on Thursday after beating her compatriot Christina McHale, maintaining her bid for a fourth tournament crown.
"Serena has great motivation to win. She still has big goals and wants to beat as many records as possible because she thinks she can," said Mouratoglu.
"But if in the future she were to realise she was no longer the best, I think that will be the moment she retires."
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