Novak Djokovic began his Australian Open title defence with a four-set win over Jan-Lennard Struff, while six-time champion Roger Federer claimed a comfortable victory at Melbourne Park.
World number two Djokovic overcame a third set wobble to beat Germany's Struff 7-6 (7-5) 6-2 2-6 6-1.
Djokovic is a seven-time champion in Melbourne and is bidding for his 17th Grand Slam title overall.
Swiss Federer earlier breezed past American Steve Johnson 6-3 6-2 6-2.
It is the first time that Djokovic has dropped a set in the opening round of the Australian Open since 2006.
However, he saw off a tiring Struff in two hours 20 minutes to set up a second-round meeting with either Japan's Tatsuma Ito or Indian lucky loser Prajnesh Gunneswaran.
Umpire's warning was terrible call - Shapovalov
Denis Shapovalov, 20, lost his cool in a 6-3 6-7 (7-9) 6-1 7-6 (7-3) defeat by Hungary's Marton Fucsovics.
The Canadian 13th seed was warned by the umpire for throwing his racquet down and responded: "I'm not breaking any rules.
"It's my racquet I can do whatever the hell I want with it. What are you talking about, I didn't break it.
"If I broke it, give me a code, 100%. I didn't break my racquet. It was a terrible call, do your job."
The rules state that players cannot "violently or with anger hit, kick or throw a racquet" within the tournament site.
Speaking after the match, Shapovalov admitted he had been nervous during the match but said the call from the umpire was "horrible".
"I'm going to get fined for whatever the code was and he gets off the hook because he makes a call because he feels like it," he said.
"I think it was a terrible call. The racquet was still intact and he gave me a warning because I did it two or three times."
Federer win 'sets up season nicely'
Third seed Federer, playing his first competitive match since the ATP Finals in November, needed just one hour 21 minutes to beat Johnson.
The 38-year-old, who is chasing a 21st Grand Slam title, will face the winner of French qualifier Quentin Halys against Serbia's Filip Krajinovic in the next round.
"I am very happy. It is nice to be back here in Australia. I felt really good, so it was a really good first round for me," Federer said.
"I trained really hard [between seasons], needed some vacation. I am just so happy I didn't have any setbacks. That sets you up nicely for the season."
Earlier, Greece's Stefanos Tsitsipas, who ended Federer's title defence in Melbourne in 2019, made light work of his first-round match against Italy's Salvatore Caruso.
Sixth seed Tsitsipas breezed past his 95th-ranked opponent 6-0 6-2 6-3 and will face Germany's Philipp Kohlschreiber next.
American Sam Querrey also progressed with a 6-3 6-4 6-4 victory over Croatia's 25th seed Borna Coric, while there were wins for Italian eighth seed Matteo Berrettini and Argentine 22nd seed Guido Pella.
A number of first-round matches that were set to take place on the outdoor courts were cancelled because of rain.