London - Roger Federer kept alive his hopes of a seventh ATP Finals crown on Tuesday, beating Italian debutant Matteo Berrettini 7-6 (7/2), 6-3 at London's O2 Arena.
Both players were solid on serve in the first set, with Federer struggling to make inroads against the eighth seed.
The Swiss, clad in black, broke immediately at the start of the second set to leave the Italian with a mountain to climb.
The 20-time Grand Slam champion said he had remained calm and tried to stay positive after the Thiem defeat.
"Not everything was bad (in the Thiem match) but of course if you over-analyse it, all of a sudden it can be," he added. "I was ready, I was prepared today and that's what matters the most."
On Monday, top seed Nadal lost his opener in Group Andre Agassi to defending champion Alexander Zverev while Stefanos Tsitsipas beat Daniil Medvedev.
Group Jonas Bjorkman
Lukasz Kubot/Marcelo Melo (POL/BRA x2) v Raven Klaasen/Michael Venus (RSA/NZL x5)