Serbia's Novak Djokovic celebrates after defeating Italy's Matteo Berrettini in their ATP World Tour Finals singles tennis match at the O2 Arena in London on Sunday. Photo: Alastair Grant/AP

LONDON - Novak Djokovic began his campaign to reclaim the world number one ranking with a comfortable 6-2, 6-1 win over Italy's Matteo Berrettini on Sunday at the ATP Finals in London. 

After an even start through the first four games, the 32-year-old Serbian broke Berrettini's serve twice to take the first set lead inside half an hour. 

Berrettini, who had never faced Djokovic before, crumbled through errors and was broken three more times as Djokovic sealed his victory with an effortless forehand in 63 minutes. 

It was the start Djokovic needed if he is to take back the world number one ranking from Rafael Nadal, who leads by 640 points. 

The 16-time grand slam winner must at least reach the final at London's O2 Arena with two round robin successes if he is to overtake Nadal and equal Pete Sampras' record of six year-end number one finishes. 

Djokovic sits at the top of the Bjorn Borg group after his latest triumph, which also features Roger Federer and Austria's Dominic Thiem, and they will play later on Sunday.