Roger Federer insisted that he is "not allowed to lose anymore" after he suffered a loss in his ATP Finals' opening match against Dominic Thiem on Monday.

With two more Round-Robin matches left, Federer - who is a part of the Björn Borg group also involving Novak Djokovic and Matteo Berrettini - needs to win those to progress into the semi-finals.

The six-time ATP Finals champion has decided to treat this week as a knockout tournament.

"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 past 20 years, so from that standpoint, there is nothing new," the Swiss Master, who last won the ATP Finals in 2011, added.

Federer also said the start against Thiem did not help him as he had a few hiccups and that probably pushed him on the "back foot."

"I thought he played well. The start definitely didn't help. That put me on the back foot. But I recovered well and I thought the match was pretty even for a long period of time. I felt that I had my chances. I didn’t feel like I was outplayed or anything. Just maybe those first-match hiccups, not hitting your spots on the serve when you need to, getting into trouble early in the service games, which maybe doesn't happen later on in the tournament," the 38-year-old legend added.

Roger Federer
Roger Federer of Switzerland reacts during his match against Kei Nishikori of Japan during Day One of the Nitto ATP World Tour Finals at The O2 Arena in London, England, Nov. 11, 2018. Clive Brunskill/Getty Images

Even though Federer's game kept fluctuating against Thiem, the crowd keep their support intact for the former champion. After multiple Wimbledon and ATP Final titles, the crowd in London now associates Federer as one of them.

Despite beginning his ATP Finals 2019 campaign with a loss, Federer was optimistic as he said the support he receives at O2 Arena in London can keep him motivated and push him toward a seventh title there.

"I thought the atmosphere was great. I wish I could have given them a bit more something to celebrate tonight. I felt like the fans were waiting and they were ready to go… Maybe getting that break in the second set would have done exactly that, but it's a wonderful event here. I'm happy I’ve played here so many times so I could enjoy the crowds. I'm looking forward to the next two [matches]," Federer told ATP after his 7-5, 7-5 loss to Thiem.

Federer is set to next face Berrettini, who also lost his opening fixture to Djokovic.