Roger Federer's pursuit of a record-extending seventh ATP Finals title got off to a poor start when the Swiss lost to Austrian Dominic Thiem in his opening group match.
From the moment Federer dropped his opening service game he never seemed entirely comfortable and Thiem took full advantage to win 7-5, 7-5 and claim his third success over Federer this year.
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Federer recovered the early break but a couple of lazy strikes at 5-5 cost him his serve again before Thiem closed out the opener in confident fashion.
Thiem, beaten by Federer in the group stage last year, increased his power in the second set and pounced again to break at 5-5.
Federer saved one match point in the following game and had a break point but there was no escape as Thiem pummelled away a forehand to claim victory.
Fans on social media thought Federer looked unusually ‘awful’ during the match, despite him fighting till the end.
This was epitomised when he missed an overhead smash, which the Swiss maestro would normally put away.
Thiem joined Novak Djokovic as the winner on the day after the Serb earlier on Sunday crushed tournament debutant Matteo Berrettini.
Thiem, who now leads Federer 5-2 on head-to-head, will face Djokovic on Tuesday while Federer will be up against Italian Berrettini.
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"It was a great performance and special to get my first win in an opening group match in the Finals," Thiem said.
"It's overwhelming to enter this arena and we had a good and close match. It's always special to beat Roger as he is a legend."
Federer also lost his opening group match last year to Kei Nishikori but recovered to reach the semi-finals.
Djokovic flew out of the traps in his quest for a sixth end of season title and a return to the top of the rankings, cruising past Berrettini 6-2 6-1.
However, still found a way to berate himself.
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.
"There's no other reasonable explanation (to that reaction) than trying to play as perfect as possible," Djokovic said.
"But I guess that's me, you know."
Berrettini became the first Italian to play in the season-ending showpiece since 1978 but was given a harsh lesson by the 16-time grand slam champion at London's O2 Arena.
Djokovic must at least win two group matches and reach the final to have any chance of overtaking Rafael Nadal and matching Pete Sampras's record of ending six separate years as world No.1.
The other four-man group features Nadal, defending champion Alexander Zverev, Stefanos Tsitispas and Daniil Medvedev.