The Bjorn Bjorg group features Djokovic, Roger Federer, Dominic Thiem and Matteo Berrettini.
Matteo Berrettini has had an outstanding season as he has managed to establish himself as an elite player on the ATP tour.
If the Serbian does claim the year-end number one ranking for a sixth time, he would match the record of Pete Sampras and victory at the end-of-season event would equal Roger Federer´s mark.
The 33-year-old said he was happy to be in London after being forced to pull out of last week's Paris Masters at the semi-final stage with an abdominal strain, but admitted he was not serving at full tilt ahead of his opening match against defending champion Alexander Zverev.More news: Prince Harry, Meghan, Prince William And Kate All Watch Remembrance Service
It was certainly the same Djokovic that the O2 Arena crowd has become used to seeing.
Federer squandered an early chance to break in the second set and the games went with serve until the 11th game again, when Thiem broke to love after the third seed dumped a forehand into the net.
Leading 4-0 in the second set, Djokovic hit a backhand wide to concede a break to the Italian, and then angrily turned toward his player's box and shouted out his frustration.
Federer and Djokovic will meet later in the week in a repeat of this year's epic Wimbledon final, with two players advancing from the four-man group. The seven days there will also be five ATP Challenger events: the Oracle Challenger Series - Houston (USA, combined ATP Challenger / WTA 125K, on-courts), the JSM Challenger of Champaign-Urbana (USA, on indoor hard-courts), the Tali Open (Finland, on indoor hard courts), the Sparkassen ATP Challenger (Italy, on indoor hard courts) and the KPIT MSLTA Challenger (India, on hard-courts).More news: Impeachment testimony also gives Trump a quid pro quo defence
"The ball is not jumping a lot, so it's tough to mix, especially my forehand", Berrettini said.
Dominic Thiem continued his dominance of Roger Federer this year with a straight-sets victory on the opening day of the ATP Finals.
"But I recovered well, and I thought after that the match was actually pretty even for a long period of time".
Djokovic has a chance to overtake Nadal for the year-end No. 1 ranking with a strong showing in London.More news: Mercury’s rare Sun transit will happen on Monday: Here’s how to watch
Nadal, Daniil Medvedev, Stefanos Tsitsipas and Zverev are in the Andre Agassi group. He trails the Spaniard by 640 ranking points, with a maximum of 1,500 points up for grabs.
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