WATCH: Roger Federer practices in London ahead of Sunday start

Roger Federer is officially in the building. The 20-time Grand Slam champion is practicing inside the O2 Arena in London as he gears up for the Nitto ATP Finals, which starts on Sunday.

The Swiss is a six-time champion at the year-end championships and is the No. 2 seed. He's in Group Lleyton Hewitt with Kevin Anderson, Dominic Thiem and Kei Nishikori. Group Guga Kuerten features top seed Novak Djokovic, Alexander Zverev, Marin Cilic and John Isner.

Federer has a 4-1 winning record against Anderson, but the South African stunned him at Wimbledon earlier this year, coming from two sets down to win 13-11 in the fifth. The 37-year-old has a comfortable 7-2 record against Nishikori, while Thiem actually leads in their head-to-head matches, 2-1—though they haven't played each other since 2016.

Defending champion Grigor Dimitrov did not qualify this year, while world No. 2 Rafael Nadal withdrew due to an abdominal injury and ankle surgery.

Federer will be the night match on Sunday against Nishikori, while the first singles match of the tournament will see Anderson face off against Thiem.

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