Novak Djokovic ruthlessly took Matteo Berrettini apart at the ATP Finals on Sunday as he began his quest to reclaim the number one ranking by winning his opening group match 6-2 6-1.
Roger Federer already faces an uphill task to win the title for the first time since 2011, however, after being outplayed by inspired Austrian Dominic Thiem, losing 7-5 7-5.
Thiem had never won his opening group match in his three previous appearances but took full advantage of an unusually sluggish Federer to make a flying start.
While six-time champion Federer will fancy his chances of steadying the ship against Berrettini on Tuesday, he will likely need to beat Djokovic on Thursday to reach the semi-finals.
Only once in 16 previous appearances has Federer not survived the group stage, back in 2008.
Berrettini, the first Italian to play in the season-ending showpiece since 1978, was given a harsh lesson by 16-time Grand Slam champion Djokovic at the O2 Arena.
The 32-year-old Djokovic began the match 640 points behind Rafael Nadal in the race to end the year on top of the rankings, but victory chipped that advantage down to 440.
Djokovic must at least win two group matches and reach the final to have any chance of matching Pete Sampras’s record of ending six separate years as world number one.
Spaniard Nadal, who has concerns over an abdominal injury sustained at the Paris Masters, plays his first group match on Monday against defending champion Alexander Zverev.