Patrick Mouratoglou (picture) will be staying as Serena Williams’ coach but admitted the world number one is going through “a difficult period”.

Williams has struggled at grand slams this year, failing to progress beyond the fourth round, and was beaten in round three at Wimbledon by France’s Alize Cornet.

But it was Williams’ performance in the doubles at the All England Club alongside sister Venus that really gave cause for concern.

The American appeared barely able to stand and was persuaded to retire from the match after serving a whole game of double faults.

The official explanation was that Williams was suffering from a “viral illness” but that was met with scepticism and the plot thickened when Mouratoglou revealed he had not seen her for two days prior to the match.

The pair have never confirmed anything beyond a professional relationship but it has been an open secret within the sport, and Williams’ state led to speculation of a rift between them.

Mouratoglou insisted that is not the case and that Williams is training hard and determined to rediscover her best form ahead of next month’s US Open.

The Frenchman told France’s Tennis Magazine: “Serena is clearly going through a difficult period. But after Wimbledon we had many hours of discussions and we have restarted our work.

“Today, she is in a state of mind where she wants to train hard to come back strong. At no time was there any question of our collaboration ending.”

Mouratoglou refused to discuss whether their personal relationship has changed, but said cryptically: “There were complicated things and it is true that Serena is in a difficult phase, but I’ll leave it there.”

Williams is due to return to action at the Bank of the West Classic at Stanford University beginning on July 28.