What is the fine-grained topic of the following text: Novak Djokovic, who returns to the O2 as world number one, won the title in London from 2012-15, and while the venue has served him well, he believes a move away from London in 2021 must now be considered by the men’s Tour. Only return the topic.
Novak Djokovic's opinion on moving the men’s Tour away from London in 2021.