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 a 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.