“But overall just happy. I’m pleased about the level that I played today,” added Alcaraz, who was playing his first competitive match for nine weeks.
Alcaraz has won the US Open, Wimbledon and French Open, but Melbourne is the one Grand Slam missing from his impressive resumé.
The furthest he has gone at the opening major of the year is the quarter-finals and he has made it clear that dethroning rival Jannik Sinner as champion is his main aim for 2026.
If he does so, the 22-year-old would surpass compatriot Rafael Nadal to become the youngest man to complete the career Grand Slam.
Earlier, in steamy conditions approaching highs of 30C, Zverev threatened to implode in losing the first set to Canada’s Gabriel Diallo.
But the 28-year-old German recovered to win 6-7 (1/7), 6-1, 6-4, 6-2 and next meets Australian Alexei Popyrin or Alexandre Muller of France.
Asked how he reset from his first-set wobble, the third seed said: “I was thinking it can’t get worse than that.”
British qualifier Arthur Fery scored the first big upset by taking down 20th seed Flavio Cobolli of Italy, 7-6 (7/1), 6-4, 6-1.
Novak Djokovic, who is chasing a record 25th major title, begins his title assault on Monday, as does three-time runner-up Daniil Medvedev.
Organisers said that a record 100,000 people came through the gates, although there were complaints about long queues in the heat.
Underlining how fierce it was on court, a ball girl collapsed during the match between Zeynep Sonmez and Ekaterina Alexandrova, with both players rushing to help her.
Sabalenka began her bid for a third Australian Open title in four years with a patchy start before coming good in the second set.
The world No 1, stunned by Madison Keys in the 2025 final, ultimately had too much power, guile and quality for French wildcard Tiantsoa Rakotomanga Rajaonah.
But the Belarusian took time to find her touch, making a series of unforced errors before taming the spirited 20-year-old 6-4, 6-1 on Rod Laver Arena.
“It is always tricky playing someone young, someone you don’t know, and a lefty [left-hander],” said the top seed and title favourite.
Sabalenka said she had felt added pressure with tennis legends Roger Federer and Rod Laver courtside.
“I hope you guys enjoyed watching me play, I hope you enjoyed it even a little bit,” she said in her on-court interview, addressing the duo.
Sabalenka faces Chinese qualifier Bai Zhuoxuan next.
Title rivals Iga Swiatek and Coco Gauff play on Monday.
Also safely through was seventh-seeded Italian Jasmine Paolini as she outclassed Belarusian qualifier Aliaksandra Sasnovich 6-1, 6-2.
Elina Svitolina, the Ukrainian 12th seed, eased past Cristina Bucsa of Spain 6-4, 6-1.
But seeded fellow Ukrainians Dayana Yastremska and Marta Kostyuk were dumped out early, as was the American great Williams.
The seven-time major champion was the oldest woman to play in the history of the Australian Open.
She showed glimpses of what once made her world No 1, but ultimately fell after an epic battle with an opponent more than 20 years her junior.
Olga Danilovic of Serbia outlasted the veteran 6-7 (5/7), 6-3, 6-4 over a gruelling two hours 17 minutes.
“It was such a great game, such a great moment. The energy from the crowd was amazing. That lifted me up so much,” Williams said.
