"Growing up, I've looked up to him,'' said Giron, a 28-year-old from the U.S. "He's one the absolute legends of the game.''
Nadal won the Australian Open in 2009 and is the only past champion in the men's bracket after Djokovic's late withdrawal Sunday because he was forced to fly out of the country when a court upheld the cancellation of his visa. Federer is skipping the tournament to continue recovering from right knee surgery.
Monday was Nadal's first appearance at a major since he lost to Djokovic in the semifinals of the French Open. He missed Wimbledon because of fatigue, skipped the US Open because of a painful left foot and, after recovering from a breakthrough case of COVID-19 in December, started 2022 with a title in a tuneup tournament in Melbourne.
"Honestly, it's been very tough moments and there are still doubts,'' Nadal said. "But here I am and I can't be happier to be back in this amazing stadium. It's fantastic.''
During his on-court interview with Jim Courier following Monday's match, Nadal discussed his challenging experience with the coronavirus.
"The symptoms haven't been nice," Nadal said. "I have been four days in bed and then three more days just destroyed physically. After that I started to feel a little bit better."
The Spaniard said he was able to have "one or two practices at home" once he received a negative PCR test before coming to Australia to play in a lead-in event. He said he feels better now but wasn't sure if the fatigue he was feeling was related to COVID-19 or the lack of recent match play.
"I don't know if sometimes you're a little bit more tired because of it or because I haven't been on the tour for the past six months," he told Courier.
Nadal is bidding to become the second man in the Open Era -- and only the fourth man -- to win each of the four Grand Slam singles titles twice. Djokovic completed the double by winning his second title at Roland Garros in 2021. Australians Roy Emerson and Rod Laver are the only other men to have won each Grand Slam on two or more occasions.
Djokovic had been scheduled to play Monday night's last match in Laver, following No. 1-ranked woman Ash Barty's contest, but instead the 34-year-old Serbian player was on a flight that landed in Dubai.
Showing evidence of vaccination is a requirement for anyone -- players, coaches, fans, others -- entering Melbourne Park for the tournament.
A security official with a loudspeaker reminded people lining up outside the complex early Monday to have proof of vaccination ready for inspection and added: "Oh, a ticket would be handy, too!"
Fans were taking photographs at the main stadium in front of an image of Djokovic holding up the trophy he won each of the past three years.
In other results Monday on the men's side, No. 31 Carlos Alcaraz opened with a 6-2, 6-2, 6-3 win over Alejandro Tabilo. No. 17 Gael Monfils defeated Federico Coria in straight sets, while No. 14 Denis Shapovalov followed up his win with Canada in the ATP Cup with a victory over Laslo Djere of Serbia 7-6 (3), 6-4, 3-6, 7-6 (3).
The Associated Press contributed to this report.