French Open finalist Stefanos Tsitsipas headed a long list of seeds to exit the Paris Masters on Wednesday as an arm injury forced him to retire from a match for the first time in his career.

The 23-year-old Greek was clearly out of sorts from the beginning of his second-round match with Australian lucky loser Alexei Popyrin and gave up the ghost when trailing 4-2 in the first set.

Tsitsipas, ranked three in the world, said he was more concerned about ensuring he is fit for the season-ending ATP finals in Turin.

"I haven't retired once in my life, and it was something that I had to do today," the Greek said.

"Also I'm trying to be cautious for the next tournament which is the most important one for me."

Tsitsipas refused to divulge what the exact nature of the problem was, but said it had been an issue for a while.

Unlike Tsitsipas, fourth seed Alexander Zverev saw out his match against Serb Dusan Lajovic, despite struggling with a painful neck, winning in straight sets 6-3, 7-6 (7/5).

Earlier, US Open semifinalist Felix Auger-Aliassime slumped to a straight sets defeat to Dominik Koepfer, who had knocked out former world number one Andy Murray in the first round.

The 21-year-old Canadian's disappointment was compounded by the fact the 6-3, 7-5 defeat also ended his hopes of reaching the ATP Finals.

The ninth seed had required three sets on Tuesday to overcome Italian qualifier Gianluca Mager but there was no way back against lucky loser Koepfer and Auger-Aliassime's cause was not helped by eight double faults.

"I fought against myself some times," he said. "I was motivated to have a good week here, so of course I'm greatly disappointed."

Taylor Fritz, who knows what it is like to lose to Koepfer having fallen to him in this year's French Open, sent fifth seed Andrey Rublev packing 7-5, 7-6 (7/2).

A broad grin spread over the big-serving American's face, a stark contrast to the grimace of pain as he exited Roland Garros in a wheelchair having injured his knee in his defeat by Koepfer.

Fritz plays the in-form British number one Cameron Norrie in the last 16 after he beat another powerful American, Reilly Opelka, 6-3, 6-4.

'It is cool'

There was joy for the sizeable home crowd as French qualifier Hugo Gaston, ranked 103rd in the world, beat Spanish 12th seed Pablo Carreno Busta 6-7 (3/7), 6-4, 7-5.

Gaston, 21, refused to compare his victory to that of last year's French Open when he beat Stan Wawrinka, but enjoyed the moment all the same.

"I had nothing to lose, it was crucial that I play my natural game so I had no regrets at the end of the match," he said.

"It is cool to have the chance to play another game here."

Gaston will play fast-rising Spanish 18-year-old Carlos Alcaraz after he ousted another seed, the in-form Jannik Sinner.

Alcaraz, who reached the US Open quarter-finals before retiring with a thigh injury against Auger-Aliassime, saw off the eighth-seeded Italian 7-6 (7/1), 7-5.

"I knew that he was going to be a top player already when I played him in 2019 on the clay in the Challenger Tour," said Sinner of Alcaraz.

Alcaraz and indeed Sinner will aspire to challenge number one Novak Djokovic and number two Daniil Medvedev in the years to come.

Medvedev has dreams of toppling Djokovic, the man he calls his friend, as year-end world number one.

The 25-year-old Russian -- who dashed Djokovic's dreams of a Grand Slam sweep by beating him in September's US Open final -- eased to a routine 7-5, 6-4 win over Ilya Ivashka of Belarus.