World No. 1 Jannik Sinner made it two wins out of two at the Association of Tennis Professionals (ATP) Finals as he fought off Taylor Fritz in a high-octane clash inside the Inalpi Arena on Tuesday.

In a repeat of September's U.S. Open final, home favorite Sinner delighted a raucous crowd with a 6-4, 6-4 victory in the best match of the tournament so far.

American world No. 5 Fritz went toe-to-toe with Sinner throughout a contest ultimately decided by slender margins with one break of serve in each set.

Sinner made the decisive break in the opening set when Fritz served at 5-4, bringing up a set point with a disguised drop shot and then converting at the first opportunity.

Fritz refused to back down in the second set and had a glimpse of a chance at 3-3 when Sinner fell 30-0 down on serve. Sinner averted that danger though with some fearless tennis and then exerted the pressure when Fritz served to stay in the match, clinching victory in one hour 40 minutes.

Sinner, bidding to become the first Italian to win the prestigious year-ending tournament having reached the final last year, tops the Ilie Nastase Group with two straight sets wins and looks odds on to reach the semi-finals.

Fritz is tied on one win with Russian Daniil Medvedev who earlier beat Australian debutant Alex de Minaur to keep himself in contention for a semifinal spot.

"It was a very tough match, we both knew exactly what to expect," Sinner said after his 67th victory of the season. "He was very aggressive. I managed to serve very well in the crucial moments. I'm happy how I handled the tough situations like at 3-3 in the second set when the momentum could have changed if he had broken me."

Fritz will be hopeful of beating De Minaur in his last group match to reach the semifinals although world No. 4 Medvedev, the 2020 champion, has it all to do against Sinner.

Medvedev said he was ready to pack his bags after losing to Fritz on Sunday, but produced a serving masterclass to overpower De Minaur 6-2, 6-4.

The Russian's tennis and attitude on Tuesday were in stark contrast to Sunday when he looked distracted and disinterested as he earned a penalty point for smashing a courtside microphone.

This time he was fully focused on the job in hand as he made only 10 unforced errors and conceded only nine points on his serve as he extended his record over De Minaur to 7-3.

Medvedev had been booed by fans during his opening loss and celebrated beating De Minaur by putting his fingers in his ears.

"I went into this match blocking the noise, even from myself," Medvedev said. "I really didn't care what was happening on the court, I just tried to play, and it was a good feeling. The more popular you become, the more fans you get, the more haters you get, the more attention you get. Sometimes even the good noise can make you off balance."