World number two Rafael Nadal began his bid for a men's record 21st Grand Slam title with a straight-set victory over Laslo Djere at the Australian Open.
The Spaniard did not play in the ATP Cup last week after struggling with a back injury, but he moved well in a 6-3 6-4 6-1 victory over the Serb.
He will play American world number 177 Michael Mmoh in the second round.
Russian fourth seed Daniil Medvedev and compatriot Andrey Rublev also advanced on the second day at Melbourne Park.
Fifth seed Stefanos Tsitsipas cruised through against Gilles Simon, seeing off the 36-year-old Frenchman 6-1 6-2 6-1 in one hour 32 minutes.
However, 13th seed David Goffin squandered four match points as he was stunned in five sets by Australian wildcard Alexei Popyrin.
Popyrin, who was one of the 72 players to undergo 'hard' quarantine before the Grand Slam began, fought back to beat the Belgian 3-6 6-4 6-7 (4-7) 7-6 (8-6) 6-3.
Spanish 12th seed Roberto Bautista Agut also suffered a surprise defeat, losing 6-7 (1-7) 6-0 6-4 7-6 (7-5) to Moldovan world number 85 Radu Albot.
Nadal, who reached the final in Melbourne in 2019, did not have to exert himself too much in his win over Djere.
His only troubles came when serving for the set - he was broken to love at 5-2 in the first and was forced to stave off break point at 5-4 in the second.
Greece's Tsitsipas had the Rod Laver Arena crowd laughing - and jokingly booing - following his quick victory over Simon when he said: "I wasn't expecting it to come so easy."
The 22-year-old added: "I feel like I messed it up already. I think I played a spectacular match from the beginning to the end.
"That was the last thing I was expecting, playing on Rod Laver and everyone laughing at me!"
Meanwhile, Medvedev extended his winning streak to 15 matches with a 6-2 6-2 6-4 win over Canada's Vasek Pospisil.
Eighth seed Rublev beat Yannick Hanfmann of Germany 6-3 6-4 6-4 to continue his own good form.
Italian ninth seed Matteo Berrettini saw off South Africa's Kevin Anderson 7-6 (11-9) 7-5 6-3, while Russia's Karen Khachanov overcame a second-set dip to beat Australian Aleksandar Vukic 6-3 6-7 (4-7) 7-6 (7-2) 6-4.
Australian prospect Alex de Minaur also progressed, with the 21-year-old 21st seed coming through 7-5 6-1 6-1 against American Tennys Sandgren.