Britain's Andy Murray says he needs to continue to improve if he is to remain world number one.
The 29-year-old Wimbledon champion replaced Novak Djokovic at the top of the rankings at the end of 2016.
Murray returns to Grand Slam action at the Australian Open in Melbourne next week, where he has been runner-up on five occasions.
"The reality is, in sport, that things obviously keep moving on, the game will get better," the Scot said.
"I'll obviously get older, the young guys will continue to improve, and also Novak, Roger Federer, Stan Wawrinka, Rafa Nadal and all the guys at the top are still going to be wanting to get there.