Having split from Rainer Schuttler in July, Angelique Kerber will be bringing in Dieter Kindlmann as her new coach.
Croatia's hopes of retaining the Davis Cup are over after they were beaten by hosts Spain in Madrid.
Spain overcame Russia in their Davis Cup Finals opener as Rafael Nadal claimed a singles win.
He may have won 20 grand slams, but Roger Federer is still motivated, even at 38.
The revamped Davis Cup Finals began with defeats for defending champions Croatia and a talented Italy team.
The history-making Stefanos Tsitsipas surprised himself with the way he blew Dominic Thiem away in the second set of a classic final.
After winning the ATP Finals, Stefanos Tsitsipas feels grand slam success is not far away.
Stefanos Tsitsipas and Dominic Thiem served up a thriller at the O2 Arena and it was the Greek who came out on top to make history.
Rafael Nadal is second only to Roger Federer on the men's grand slam all-time list - but his uncle says he could have been clear at the top.
There will be a first-time ATP Finals champion on Sunday after Dominic Thiem ended Alexander Zverev's reign in London.