On the verge of an nearly flawless performance against Matteo Berrettini at the ATP Finals, Novak Djokovic still found a reason to berate himself.
The 32-year-old needs to reach the final of the season-ending showpiece to reclaim the number one spot from Nadal and match Pete Sampras's incredible record of finishing six separate seasons as the world's number one player.
"It feels great to be back in, earlier this year I had a great stay in London, played well at Wimbledon", Djokovic told BBC Sport.
Berrettini, the last player to qualifier for the finals, walked towards the wrong exit after Djokovic's win was sealed on a first match point, and given the beating he had just taken, it was perhaps no surprise the Italian seemed slightly shocked.
From 2-2 in the first set, Djokovic ran through the next eight games for a commanding 4-0 lead in the second set, until the Italian halted his opponent's charge with a break of his own. But Djokovic, who won four straight titles at the O2 Arena between 2012 and 2015, broke right back and then clinched the victory with an easy forehand putaway at the net.
Matteo Berrettini of Italy plays a backhand during his debut against Novak Djokovic at the ATP Finals.
"It's not easy for Matteo, playing his first tour finals match, I knew he'd be a bit more nervous at the beginning, I tried to use my experience".
Djokovic, four times a victor of the title since the tournament moved to London in 2009, had never played Berrettini before but quickly got to grips with the aggressive style of the 23-year-old Italian.
It mattered little, however, Djokovic getting the better of his opponent's serve once more before rounding off the win with a controlled cross-court forehand. "I think I started the match pretty good, but he was just better than me today".
World number two Djokovic can overtake Nadal to become year-end number one this week in London but must reach the final to stand a chance.
Nadal, Daniil Medvedev, Stefanos Tsitsipas and Zverev are in the Andre Agassi group.
Nadal begins his campaign on Monday against defending champion Zverev.
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.