Having split from Rainer Schuttler in July, Angelique Kerber will be bringing in Dieter Kindlmann as her new coach.
Coco Gauff is a notable beneficiary as Angelique Kerber and Maria Sharapova have been forced out of the Luxembourg Open.
Naomi Osaka made it six wins in succession as world number one Ashleigh Barty and defending champion Caroline Wozniacki triumphed in China.
Only one Pan Pacific Open quarter-final was completed on Friday due to the rain in Osaka.
An expectant crowd were given an early scare before Naomi Osaka pulled through in straight sets at the Pan Pacific Open.
Angelique Kerber was knocked out in a first-round thriller at the Zhengzhou Open, but Jelena Ostapenko survived a major scare.
After losing 6-3 7-5 to Bianca Andreescu at the US Open, Serena Williams has now been defeated in her last four grand slam finals.
Serena and Venus Williams lost three games between them as the American pair cruised through to the second round at Flushing Meadows.
Venus Williams upset Kiki Bertens as five seeds fell at the Western & Southern Open.
Maria Sharapova – a five-time grand slam champion – was eliminated following a two hour, 41-minute thriller in Toronto on Monday.