The Serbian returned to the summit of the rankings after Spain's Rafael Nadal withdrew from the Paris Masters last month.

Djokovic, a 14-time Grand Slam champion, had slipped to as low as 22nd in the world in May following a six-month break due to an elbow injury.

He has returned sensationally in 2018, winning both the Wimbledon and US Open Grand Slam titles.

Nadal's abdominal injury means he will not be able to compete at the event at London's O2 Arena.

The event is for the top eight players and with Argentina's Juan Martín del Potro also withdrawing it has left two spaces which Japan's Kei Nishikori and American John Isner have filled.

Djokovic is playing in the group named after Brazilian player Guga Kuerten and is joined by Germany's Alexander Zverev, Croatia's Marin Čilić and Isner.

In the group named after Australian star Lleyton Hewitt, Switzerland's 20-time Grand Slam winner and second seed Roger Federer is the headline name.

He is joined by Nishikori, Austria's Dominic Thiem and South Africa's Kevin Anderson.

Bulgarian Grigor Dimitrov is not back to defend his title on the hard courts in the British capital, while a doubles tournament will also take place.

The top two in each group will reach the last four.

"Being number one is probably the ultimate challenge in our sport and the pinnacle of the season," Djokovic said.

"It's extra special this season because of the whole journey I've been through the past 15 months, particularly the last eight months, after surgery in February.

"Quite incredible, not just that I was ranked 22. 

"I never thought it was impossible but I thought it might take more time. 

"It turned out to be a perfect five months."