The top-ranked Nadal is chasing his 20th major singles title, aiming to equal Roger Federer's men's record. An Australian Open crown this year will also give him at least two career titles in each of the four Grand Slams. He won the Australian title in 2009 and has lost four finals since then at Melbourne Park.
Nadal said after his match that he isn't concerned about winning his 20th major.
"I don't care about 20 or 15 or 16. I just care about [trying] to keep going, keep enjoying my tennis career. It's not like 20 is the number that I need to reach. If I reach 20, fantastic," Nadal said, raising his hands in the air. "If I reach 21, better. If I [stay at] 19? Super happy about all the things that I did in my tennis career, no?"
Nadal, 33, dropped a service game once in each of the first two sets but was too dominant for the 73rd-ranked Dellien, who was playing his first match at the season's first major and had played only five previous matches in the main draw of a major.
"It's a positive start," Nadal said. "What you want in the first round is to win, and straight sets is better."
Tsonga, 34, won the first set in a tiebreaker before losing the next two 6-2, 6-1, calling for a trainer and then notifying the chair umpire that he could not continue.
"I have a back problem," Tsonga said later. "It's annoying, annoying me when I play since I arrived from Doha; I've got some pain.
"So, yeah, it's difficult for me to deal with it for the moment. It's new. I will go home and see with my doctors what I have to do exactly and think about it."
Popyrin recalled being motivated by Tsonga's run to the 2008 final at Melbourne Park, which ended in a loss to Djokovic.
"I never felt like this after a match. He was my hero as a kid," said Popyrin, 20. "When he made the finals here, I was in my living room jumping around like a crazy kid. Seeing him retire in this match is just painful."
A native of Australia, Kyrgios kick-started the tennis fundraising campaign that has raised millions of dollars for victims of the devastating wildfires on the continent by promising at the ATP Cup to donate $200 for every ace he serves this month. Against Sonego, he served 14.
Kyrgios' philanthropic effort has garnered the respect of one of his biggest critics.
John McEnroe told Kyrgios, 24, he is proud of him and has promised to donate $1,000 to the bushfire recovery effort for every set Kyrgios wins at the Australian Open.
After torrential rain hit Melbourne Park on Day 1, organizers had to move dozens of matches to Tuesday.
No. 12-seeded Fabio Fognini lost the first two sets Monday against Reilly Opelka but rallied Tuesday to win 3-6, 6-7 (3), 6-4, 6-3 7-6 (5), including a 10-point tiebreaker in the final set. Milos Raonic returned to play two games to finish off Lorenzo Giustino 6-2, 6-1, 6-3.
Sinner, the Next Gen ATP Finals champion, had to win only two games on Day 2 to complete his first match win at a Grand Slam event.
"I was up in the score, so it was a little bit easier for me," Sinner said of the suspension. "Obviously, I wanted to finish yesterday. We waited here 'til 8 p.m., so it's been a long day yesterday for me, for every player.''
"It was a really tough match," the fourth-seeded Medvedev said. "Up and down from both of us."
Sascha Zverev is $10,000 lighter in the pocket, but for a good reason.
After putting away Italy's Marco Cecchinato 6-4, 7-6 (4), 6-3, the 22-year-old German said he will donate $10,000 to bushfire relief for every match he wins and will donate all of his prize money if he wins the tournament.
"I know I'm not the favorite to win this event, but if I win this event, I'll donate every single cent to the bushfires," the seventh-seeded Zverev said.
The winner of the Australian Open will receive $4.12 million (Australian).
It's Isner's first win at the Australian Open since 2017, as the American got bounced in the first round in 2018 and 2019 and five of 11 previous appearances overall.
The Associated Press contributed to this report.