Serbia's Novak Djokovic celebrates after winning his group stage match against Italy's Matteo Berrettini.
"There's no other reasonable explanation (to that reaction) than trying to play as flawless as possible", Djokovic said from London.
The tournament, featuring the year´s best eight players, is contested in a round-robin format with the best four players and doubles teams reaching the knockout semi-finals stage.
Berrettini, who is the first Italian to qualify for the season-ending event since 1978, could do nothing to best the 16-time Grand Slam victor who fully controlled the rhythm of the game.
Federer, uncharacteristically wayward, squandered an early chance to break in the second set and the games went with serve until the 11th game again, when Thiem broke to love after the third seed dumped a forehand into the net.
"But, yeah, I've got to recover, I've got make sure I play better than today, and hopefully I can win that match".
"It's a normal tournament from here on forward", added the 38-year-old, who may now have to beat Djokovic to progress.
"We had a good and close match, and it's always special to beat Roger, who is a legend", said Thiem, who impressed with his clean hitting.
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"That's how it is every week of the year for the last 20 years, so from that standpoint there is nothing new there".
"But I recovered well, and I thought after that the match was actually pretty even for a long period of time".
Even at the end of another grueling season, Djokovic won't tolerate any dips in his game as he pursues a record-equaling sixth ATP Finals title.
Nadal has reached London as World No-1 but he will have to win as many matches as possible to end the year at the same place as world number 2 Novak Djokovic is hot on his heels.The duo are separated by 640 points.
Djokovic has a chance to overtake Nadal for the year-end No. 1 ranking with a strong showing in London. That being said, this is still expected to be a tough match for Matteo Berrettini as he is playing against the number 2 seed.
Nadal has never won the ATP Finals but is hoping for a positive result which would help him seal the year-end No.1 position.
Spaniard Nadal, who has concerns over an abdominal injury sustained at the Paris Masters, plays his first group match on Monday against defending champion Alexander Zverev.