While Roger Federer tops the ATP rankings amid his remarkable renaissance, former world number one Novak Djokovic has been busy in the gym trying to return to the court.

An ongoing elbow problem has plagued Djokovic, who accepted the need for surgical intervention following his fourth-round loss to Hyeon Chung at the Australian Open last month.

The 12-time grand slam champion ended his 2017 campaign prematurely, having been forced to withdraw from the quarter-finals of Wimbledon last July.

But the 30-year-old Serbian appears to be on the comeback trail, according to his latest Instagram post.

Taking to social media, Djokovic uploaded a picture of himself training in a gym, with the accompanying caption: "Working hard over here and the sweat is real. Thank you all for the continued support!"

Djokovic's woes are in complete contrast to evergreen rival and 20-time major winner Federer.

Aged 36, Australian Open champion Federer became the oldest man to occupy the top ranking en route to claiming the Rotterdam Open.