Third seed Rafael Nadal began his bid for a 21st Grand Slam men's singles title with a straight-set win against Australia's Alexei Popyrin.

The 34-year-old Spaniard, going for a record-extending 14th triumph at Roland Garros, won 6-3 6-2 7-6 (7-3).

Serbian top seed Novak Djokovic also eased through to round two, beating American Tennys Sandgren 6-2 6-4 6-2.

Russian seventh seed Andrey Rublev is out after losing in five sets to German world number 42 Jan-Lennard Struff.

Rublev's defeat means there is one fewer obstacle for Nadal in his side of the draw as the Spaniard seeks to set an outright record for major men's singles titles - a milestone he shares with Roger Federer at present.

The Russian was the highest seed behind Nadal in their quarter, with the pair projected to meet in the last eight.

With the seedings based on the world rankings, Nadal is third behind Djokovic and Daniil Medvedev this year and has been landed with a potentially tricky draw.

For the first time, Nadal has found himself in the same half of the draw as Djokovic and Switzerland's Federer, who looked impressive on his return to Grand Slam tennis after a 16-month injury lay-off.

"I am here to try my best, and when you are the third seed, you know that you're going to have the chance to be in the same part of the draw as the number one or the number two," said Nadal.

"This time I was in the part of the draw with the number one. But there still remains a lot for me to do before thinking about the semi-finals.

"I have won my first match, something that I'm happy with and I'm focused on the second one. That's it. The other stuff doesn't worry me much now."

A bright evening under the lights for Djokovic

Djokovic lost to Nadal in last year's final but his return to Court Philippe Chatrier for Tuesday's night session was much more straightforward.

The 18-time major champion warmed up by winning a clay-court title in his hometown of Belgrade last week and started sharply against 66th-ranked Sandgren, who only has one Roland Garros win to his name.

As Sandgren started testing him more, Djokovic's demeanour became sterner and he seemed to be irritated by the floodlights.

"It was strange to play under the lights, they were very bright, to be honest," the 2016 champion said.

"After a great quality first set he started to play better and raised his level. Overall I am very pleased with the way I played and how I felt on court."

Rublev, 23, has been one of the most in-form players on the ATP Tour over the past couple of seasons, boasting the second highest number of match wins in 2021 going into Roland Garros.

That tally included a pair of victories over 31-year-old Struff in the ATP Cup and the Italian Open, only for the German to avenge those defeats on the biggest stage.

Struff won the first two sets and, after Rublev fought back to level, maintained his composure to seal his first win over a top-10 player at a Grand Slam.

Heart-warming scenes as Monfils returns to winning ways

French 14th seed Gael Monfils fought back to win 1-6 7-6 (8-6) 6-4 6-4 against Spain's Albert Ramos-Vinolas, leading to jubilant celebrations between the popular player and the small number of home fans who were allowed in.

The comeback meant 34-year-old Monfils secured only his second victory since the sport resumed last summer following the coronavirus pandemic.

Monfils, one of the tour's great entertainers, has talked openly about his struggles of playing without fans and the happiness of winning again at Roland Garros was clear to see.

Putting his hand on his heart after clinching victory, Monfils received a standing ovation as he lapped up the acclaim on a buzzing Court Suzanne Lenglen.

Gael Monfils celebrates with the French crowd on Court Suzanne Lenglen

"Having the crowd there was very big, to be honest, for me. It makes it special for me," said Monfils, who added he was "really happy" to be able to play in front of his parents again.

"It was incredible. I could feel their energy. I was missing them, I was missing that."

Elsewhere, Russian 24th seed Aslan Karatsev started with a win as he looks to back up his startling run to the Australian Open semi-finals on his Grand Slam debut earlier this year.

The 27-year-old earned a 6-3 6-4 6-4 victory over American world number 163 Jenson Brooksby.

Argentine 10th seed Diego Schwartzman - a semi-finalist last year - also progressed in straight sets, winning 6-2 6-2 6-3 against Taiwanese opponent Lu Yen-Hsun.

Ninth seed Matteo Berrettini looked on course to do the same, only for Japan's Taro Daniel to pull a set back before the Italian wrapped up a 6-0 6-4 4-6 6-4 victory.