Roger Federer said he had no more margin for error if he wanted to avoid an early exit at the ATP Finals after losing his opening group match.
The 38-year-old Swiss, bidding for his seventh title at the event but his first since 2011, struggled to handle to Dominic Thiem's aggressive game as he went down 7-5 7-5.
Only once in 16 previous appearances has Federer failed to survive the group phase, in 2008, and to avoid a repeat he will have to beat Italian Matteo Berrettini on Tuesday and most probably Novak Djokovic on Thursday.
Djokovic destroyed tournament debutant Berrettini 6-2 6-1 in the day's opening singles match at the O2 Arena.
Serb Djokovic is aiming to claw back the year-end No.1 spot from Rafael Nadal and match Pete Sampras's record of ending six years at the top of the rankings.
Thiem put constant pressure on Federer and broke at 5-5 in each set to claim his third win over the Swiss this year.
"My forecast, it's a normal tournament from here on forward," Federer said.
"Not allowed to lose anymore for me. That's how it is every week of the year for the last 20 years, so from that standpoint there is nothing new there.
"I've got to recover, I've got make sure I play better than today, and hopefully I can win that match (against Berrettini)."
The warning signs were there in Federer's opening service game which he dropped and although he hit back impressively he faltered at 5-5.
Federer had the majority of support in the packed arena but Thiem again dominated the baseline rallies.
The Austrian has flourished this year since hiring coach Nicolas Massu after a slow start to the season.
"It was a great performance and special to get my first win in an opening group match in the Finals," Thiem said.
Djokovic flew out of the traps in his quest for a sixth end-of-season title.
However, he still found a way to berate himself when he dropped serve to lead 4-1 in the second set.
"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."
Djokovic will play Thiem on Tuesday.