Roger Federer has said “I’m not allowed to lose anymore” after suffering a costly opening round-robin loss to Dominic Thiem on Sunday night at London’s O2.

Federer would have expected a tougher test against Thiem, a five-time winner on the Tour this year, and so it proved. The six-time champion was beaten in straight sets, 7-5 7-5.

It came after Novak Djokovic flew out of the traps at the ATP Finals sweeping aside O2 debutant Matteo Berrettini 6-2 6-1 earlier in the day.

Federer said: “I thought the match was actually pretty even for a long period of time. I had my chances, I felt. I didn’t feel like I was outplayed or anything. Just maybe that first-round hiccup.

“It’s a normal tournament from here on forward. I’m not allowed to lose anymore. That’s how it is every week of the year for the last 20 years, so from that standpoint there is nothing new there.”

1:29
Highlights of Roger Federer’s shock defeat at the hands of Dominic Thiem at the ATP Finals

Highlights of Roger Federer’s shock defeat at the hands of Dominic Thiem at the ATP Finals

Federer’s group-stage meeting with Djokovic, their first since July’s Wimbledon final, will take place on Thursday.

The 38-year-old Swiss will probably need to avenge that five-set defeat at the All England Club if he is to reach the semi-finals.

Djokovic and Federer will meet in a repeat of their epic Wimbledon final from this summer Djokovic and Federer will meet in a repeat of their epic Wimbledon final from this summer Djokovic and Federer will meet in a repeat of their epic Wimbledon final from this summer

The crowd were firmly on Federer’s side that day, and Djokovic is likely to face more of the same on the banks of the Thames.

“I understand why there is so much support for Roger, he is who he is, and what he has achieved and the personality that he is, he deserves that,” said Djokovic.

“Sometimes the crowd is on your side, sometimes it’s against you. It’s something throughout my career I’ve had to learn how to handle, how to accept, and how to deal with that.

“It will not be anything particularly different in terms of my reaction if that happens. I’m going to accept it and respect it, and that’s it. I’m going to try to focus on what needs to be done in order to win a tennis match.”