Serena Williams and Patrick Mouratoglou during a practice session in Wimbledon 2019. #MiddleSession

  Serena Williams and her coach Patrick Mouratoglou of the United States seen at Aorangi Park ahead of a practice session during Middle Sunday of The Championships - Wimbledon 2019   

Serena williams and Patrick Mouratoglou during a practice session ahead of the 2019 Australian Open

Serena Williams speaks to her coach Patrick Mouratoglou during a practice session ahead of the 2019 Australian Open

Serena williams and Patrick Mouratoglou in 2019 Melbourne Park. #Training

Serena Williams of The United States and her coach Patrick Mouratoglou look on during a practice session ahead of the 2019 Australian Open at Melbourne Park 

Patrick mouratoglou and Serena Williams practices on court in Wimbledon 2018

Patrick Mouratoglou watches as Serena Williams of the United States practices on court during training for the Wimbledon

Serena Williams in french open 2018. #Training #PatrickMouratoglou

Serena Williams of the United States talks to her coach Patrick Mouratoglou during a practice session
Roland Garros 2018.

Serena Williams and Patrick mouratoglou. The best moments

Serena Williams' coach Patrick mouratoglou described the American's return from maternity leave as "the biggest challenge" of her tennis life, but predicted she will be back to her best by the French Open. 

Speaking to wtatennis.com ahead of the BNP Paribas Open at Indian Wells, which will be Williams' first tournament since scoring a 23rd major at the 2017 Australian Open, Mouratoglou said the 36-year-old has ambitions of winning the three remaining Grand Slams this season: the French Open, Wimbledon and the US Open.

"Serena's target [for this year] will be to win Grand Slams," said Mouratoglou, who has worked with Williams since 2012, and is also the founder of the Mouratoglou Tennis Academy. "She knows that coming back might take some time, but her level of expectation is high as always.