Roger Federer was considered an overwhelming favourite to reach the semi-final of ATP Finals 2018 from the four-man Lleyton Hewitt group but the Swiss great came up with a drab show in his tournament opener against Kei Nishikori on Sunday.

The straight-set defeat [7-6 (4), 6-3] to the Japanese star came as a huge blow to Federer's chances of making it to the last four.

However, the six-time champion came back strong on Tuesday with a dominant performance over sixth seed Dominic Thiem in their second round-robin match and keep himself in contention to reach the semi-final for the 15th time in 16 appearances.

Federer displayed glimpses of his vintage self in the must-win match as he thrashed Thiem 6-2, 6-3. The 37-year-old broke the Austrian four times during the one-sided contest and gave the London crowd a lot to cheer with some awe-inspiring winners.

The 20-time Grand Slam champion revealed his decision to skip training and focus on rest on Monday has worked wonders for him. The seasoned campaigner admitted it was his technical ability that let him down against Nishikori and said he had decided to spend the day's break with his family ahead of the second match at the Tour Finals.

"I will do the same again tomorrow because it worked. [The] important [thing] was not my forehand or my backhand or my serve or anything. I guess it was my head. For that sometimes, you need a break," Federer was quoted as saying by ATP's official website.

Roger Federer
File photo of Roger Federer.GLYN KIRK/AFP/Getty Images

"I've been playing a lot of tennis the past two months... I saw the Nishikori match on the way back to the hotel. A good, long trip. That was positive. Had a lot of time to talk. We came to the conclusions, or the coaches thought, 'Take it easy, enjoy the day with your family, and come out happy'."

Federer had headed into the season-ending tournament on the back of a series of busy weeks. The Swiss star wasn't sure about playing Paris Masters earlier this month after his Swiss Indoors title win.

However, he joined the star-studded field and went on to reach the semi-final, in which he fell in three sets to world number one Novak Djokovic.

Federer insisted that he is confident of facing fourth seed Kevin Anderson in Thursday's crucial group-stage encounter at the O2 Arena.

Notably, the two had met earlier this year at Wimbledon, wherein Anderson clinched his first win over Federer. The South African has remained unbeaten and has all but sealed a semi-final spot but the world number three needs nothing less than a win to make it to the next round.

"I feel like I'm ready to go for the day after tomorrow [against Anderson]. I don't know what happened against Nishikori. Maybe it's the round-robin format that got to me. You don't feel like you have the knife here, like in another tournament, where if you're struggling in the first round, you know if you don't get your act together, you're home in 30 minutes," Federer said.