Novak Djokovic eased to a 6-2, 6-1 victory against Matteo Berrettini in his ATP Finals opener on Sunday as he targets a record-equalling sixth year-end No.1 ranking.
The Serbian, a five-time champion at the end-of-season event, was last week nudged off the top spot in the rankings by Rafael Nadal, who is also in the eight-man field in London.
Djokovic, who won the Australian Open and Wimbledon this season, must reach the final to stand any chance of dislodging Nadal but there are major questions over the Spaniard's fitness.
The second seed saw off the challenge of Berrettini in just 62 minutes, converting five of seven break points and out-serving his 23-year-old opponent.
Federer, Nadal and Djokovic were the top three seeds when they first competed together at the tournament way back in 2007 and are still the men to beat 12 years later.
