Novak Djokovic can take the first step towards regaining the number one ranking when he opens his title defence at the ATP World Tour Finals on Sunday.
The Serb, 29, plays Austria's Dominic Thiem in the first of the singles matches at 14:00 GMT (live on BBC Two).
Djokovic could reclaim the top ranking from Andy Murray with another successful week at London's O2 Arena.
"It's one of the things that we're all fighting for here, let's see where it takes us," Djokovic told BBC Sport.
"Being number one in the world today, in a season like this, is of course the pinnacle of the sport next to winning a Grand Slam.