TURIN, Italy - Defending champion Jannik Sinner reached the semifinals of the ATP Finals with a 6-4 6-3 win over two-times winner Alexander Zverev on Wednesday, with Ben Shelton eliminated after losing earlier to Felix Auger-Aliassime in the same group.

Italy's Sinner extended his indoor hardcourt winning streak to 28 matches, but victory over his German rival was not as comfortable as the scoreline suggests, with the world No 2 under pressure early in both sets.

"A very, very competitive match, a very close match," Sinner said. "I felt like I was serving very well in important moments. I tried to play the best tennis possible when it mattered, which fortunately went my way."

The pair, the only two previous ATP Finals champions in this year's competition in Italy, had both won their opening Bjorn Borg Group matches.

On Wednesday, Sinner faced seven break points compared to Zverev's four but pulled out aces and delightful drop shots when it counted.

Sinner made a slow start facing two break points in the opening game but found four aces at vital points to hold after nine minutes. He let slip two break points at 5-4 up before racing to the net to outwit Zverev and take the first set.

Sinner came back from 0-40 to hold his first service game of the second set and Zverev forced another break point when the Italian next served, but the champion's composure never wavered and he broke to lead 4-2, a sliced drop shot the winning point.

Zverev responded by taking a 30-40 lead the following game but Sinner held firm. At one stage a whipped backhand down the line had the German shaking his head in disbelief, and he fell to his third loss to Sinner in 17 days, while the Turin crowd rose to acclaim the Italian.

Sinner must retain his title undefeated to have any chance of ending the year as world number one, while Carlos Alcaraz needs one more match win to stay top of the rankings.

Alcaraz, with two wins from two, faces Lorenzo Musetti on Thursday, with Taylor Fritz meeting Alex de Minaur in the other match of the tournament's second Jimmy Connors Group.

Auger-Aliassime earns first win

Canada's Auger-Aliassime, who lost his opener against Sinner, came from a set down to beat Shelton 4-6 7-6(7) 7-5, to leave the American without a win after his defeat against Zverev.

Shelton powered to the opening set but Auger-Aliassime forced a decider with a tiebreak victory in the second and broke serve to convert a third match point in the final set.

The American lost his cool when failing to serve out for the first set, launching his racket in frustration when Auger-Aliassime made it 5-4, but Shelton broke again.

In the second set tiebreak, where Shelton fell and hurt his knee, Auger-Aliassime took a 3-0 lead. Shelton managed to save three set points before a double fault ended his valiant effort.

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The Canadian held break points at 2-1 up in the final set but had to wait until the final game where Shelton was guilty of gifting match points, and Auger-Aliassime did not refuse.

Auger-Aliassime will face Zverev on Friday, with a semifinal place on the line.