Third seed Daniil Medvedev survived a scare at the Australian Open as he came from two sets down to beat Emil Ruusuvuori in a match that finished at 3.40am.
Medvedev looked like he might have been in bed much earlier as the unseeded Finnish player took control.
But the Russian showed his fighting spirit to ensure that it turned into an early morning as he battled back to win 3-6 6-7 (1) 6-4 7-6 (1) 6-0, with sunrise two hours away.
Novak Djokovic’s struggles will have given heart to his rivals but few are finding the early stages of the tournament straightforward, and second seed Alcaraz needed three hours and 25 minutes to defeat Sonego 6-4 6-7 (3) 6-3 7-6 (3).
In a match full of shot-making, Alcaraz topped the highlight reel with two winners around the net post.
“I didn’t feel I had downs in the match,” said the Spaniard. “In the tie-break, he started to play an unbelievable game. I think he made some big returns after a good first serve from me.
“I think probably I could do something else in the tie-break. But the level that he played, it was really, really high.”
Sixth seed Alexander Zverev and 11th seed Casper Ruud both needed fifth-set tie-breaks to edge into round three.
Zverev looked in deep trouble down two sets to one against Slovakian qualifier Lukas Klein before recovering to win 7-5 3-6 4-6 7-6 (5) 7-6 (10/7).
“But he played extremely well. He played very, very aggressive, hitting every single ball as hard as he could from both sides, I feel like. A lot of the times I was a spectator in the match. I was just witnessing whether he’s going to hit a winner or miss.
“That’s not a nice feeling to be in, especially in the important moments, but I’m happy that I managed to win.”
Zverev next faces American 19-year-old Alex Michelsen, who knocked out last year’s quarter-finalist Jiri Lehecka.
Ruud was given a huge battle by Australian Max Purcell, who twice fought back from a set down to force a decider before the Norwegian prevailed 10-7 in the deciding tie-break.
“He’s a really tricky player and a great tennis player in my eyes, even though he plays different from others,” said Ruud.
“I’m very happy with the win. He beat me in Cincinnati. The plan was to do things better from that time, and I think I did, and that was just enough to win the match.”
Eighth seed Holger Rune is out, though, as Frenchman Arthur Cazaux won 7-6 (4) 6-4 4-6 6-3.