World number one and home favourite Jannik Sinner began his bid to win the ATP Finals for the first time with a 6-3 6-4 defeat of tournament debutant Alex de Minaur on Sunday.

Sinner, runner-up to Novak Djokovic in the prestigious year-ending tournament last year, dropped behind early in the first set but quickly hit back to dominate the contest.

The Italian, who won this year's Australian and U.S. Open titles to open his Grand Slam account, completed victory with an ace to make it eight wins out of eight against De Minaur.

Earlier in the Ilie Nastase Group American Taylor Fritz beat Russian Daniil Medvedev 6-4 6-3.

Sinner, 23, had not played a competitive match for four weeks since taking the title in Shanghai, his seventh of the season, and there was some early rust as he dropped serve to trail 2-1 in the opening set.

But he broke back immediately and his power from the baseline proved too much for De Minaur thereafter as he was roared on by the crowd in the 13,000-seater Inalpi Arena.

"Considering I hadn't played for four weeks I'm very happy," Sinner said at court side. "I started off with some unforced errors at the beginning of the match but I stayed there mentally knowing that at some point my tennis would arrive.

"It arrived quite early and started to return very well. Today I'm pleased with the win and it will give me confidence."

Sinner is hoping to cap a superb year on the court, although a cloud still hangs over the Italian who was cleared of wrongdoing by an independent tribunal after testing positive for the anabolic agent clostebol in March this year.

The tribunal, organised by the International Tennis Integrity Agency (ITIA), accepted Sinner's explanation that the banned substance entered his system from a member of his support team through massages and sports therapy.

The World Anti-Doping Agency (WADA) has since appealed the 'no fault or negligence' decision to the Court of Arbitration for Sport (CAS).

Earlier in the day former ATP Finals champion Medvedev lost his cool and risked getting defaulted because of his bizarre antics during his defeat by Fritz .

The 2020 champion lost his composure after a string of double faults in the first set as Fritz beat the Russian for the first time in his career.

Medvedev served three double faults to drop serve at 4-5 in the first set and was so incensed that he smashed his racket.

He lost his cool again in the second set and began to unravel, breaking a microphone with his racket while he was also given a point penalty. Bizarrely he later faced a Fritz serve holding the racket the wrong way round.

"I get angry, frustrated. This time completely with myself, not with anyone. Just with myself," Medvedev told reporters.

Monday's action includes Spain's Carlos Alcaraz up against Norway's Casper Ruud in the John Newcombe group while Germany's Alexander Zverev plays Andrey Rublev.