Andy Murray expects Novak Djokovic to "start playing his best tennis again" soon as the pair prepare to compete for the world number one ranking.
Britain's Murray overtook Djokovic in the rankings on Monday and needs to match his results at next week's ATP World Tour Finals to end 2016 on top.
However, the Serb has won the past four titles at London's O2 Arena.
"There's no reason to think he won't have a really good tournament," said 29-year-old Murray.
Djokovic's form has been the subject of much debate in recent months, with just one tournament win to his name since June and early exits at Wimbledon and the Olympics.