Simona Halep is a former world number one and has won two Grand Slam titlesSimona Halep will work with Patrick Mouratoglou, the long-time coach of Serena Williams, on a full-time basis.
American great Williams has been coached by Mouratoglou since 2012 but has not played since injuring her leg at Wimbledon in June.
Mouratoglou said he had missed coaching over the past eight months.
"I had a conversation with Serena, and the door opened for me, at least short term, to work with someone else," he wrote on Twitter.external-link
Former world number one Halep beat Williams to win Wimbledon in 2019.
She won her maiden Grand Slam title at the French Open the year before but has struggled in the past 12 months with injury and is now 20th in the rankings.
Mouratoglou said he initially turned down Halep's request for him to coach her before he spoke to Williams.
He has previously worked with players such as world number five Stefanos Tsitsipas and rising star Coco Gauff.
"Simona came to the Mouratoglou Academy before Indian Wells for a training block. I swung by at a few of her practices, watched her train," Mouratoglou said.
"In the last eight months, I realized how much I missed coaching. It is the passion of my life, and I still feel like I have so much to give."
Williams won 10 of her 23 Grand Slam titles with Mouratoglou as her coach, as well as Olympic singles gold in 2012 and three WTA year-end world championships.
She has fallen to 240 in the world rankings as a result of her eight-month injury lay-off.
There was speculation the 40-year-old may retire after she left her post-match news conference at the Australian Open in 2021 in tears.
Williams gave the Rod Laver crowd a long wave goodbye with her hand over her heart after her defeat by Naomi Osaka.
Asked if that was her way of saying goodbye, Williams said: "If I ever say farewell, I wouldn't tell anyone" before growing tearful and leaving the room.
She has reached four Grand Slam finals since returning from maternity leave in 2018.
