Novak Djokovic survived a huge test Wednesday to stay on track at the Australian Open but sixth seed Ons Jabeur and former champion Caroline Wozniacki fell victim to exciting young Russians.
The Serbian superstar, playing for an 11th Melbourne Park crown and a record 25th Grand Slam title, was pushed hard in the second round by home hope Alexei Popyrin but prevailed 6-3, 4-6, 7-6 (7/4), 6-3.
Djokovic also dropped a set against unheralded Croat Dino Prizmic and was again under the weather, blowing his nose regularly at the changeovers.
Despite being below par, his never-say-die attitude was on full display, saving four set points in the third set to take it to a tie-breaker that swung the momentum back in his favour.
"I haven't been playing my best, I'm still trying to find form," admitted the 36-year-old.
"Both my first and second-round opponents were really quality tennis players. I managed to find a way to win in four, that's what counts in the end.
"Hopefully I can find a way to build on this as the tournament progresses."
The hard-earned win put him into a showdown next with Argentine Tomas Martin Etcheverry, who ended Andy Murray's tournament.
Fellow defending champion Aryna Sabalenka had much easier ride, pummelling her second straight teenage qualifier into submission, beating 16-year-old Czech Brenda Fruhvirtova 6-3, 6-2.
Sabalenka has yet to be seriously tested as she bids to become the first woman to successfully defend a Melbourne title since Victoria Azarenka in 2013.
"I'm just trying to play my best. If it goes to two sets, I'm happy to win it in two sets. If it's going to be three sets, I don't care," she said. "I just try to focus on myself and fight for every point."
Fourth seeds Coco Gauff and Jannik Sinner, along with fifth seed Andrey Rublev and world number 11 Barbora Krejcikova, similarly had no trouble progressing.
Last year's beaten finalist Stefanos Tsitsipas also survived, but needed four tense sets to get past Australian Jordan Thompson in a match in which he said "the adrenaline and the rush was insane".
Veteran pair Jabeur and Wozniacki were no match for Mirra Andreeva and Maria Timofeeva, who emphatically stamped their mark on the opening Grand Slam of the year.
Andreeva, just 16, was in scintillating form to take down 29-year-old three-time major runner-up Jabeur 6-0, 6-2 on Rod Laver Arena in just 54 minutes.
"Probably it was the best match (ever)," said Andreeva, a schoolgirl who burst onto the scene when she reached the fourth round at Wimbledon last year as a qualifier.
"The first set, I didn't expect that I would play this good. Second set was also not bad. So, yes, for me it was an amazing match."
Her reward is a third-round clash with France's Diane Parry.
Timofeeva, a 20-year-old qualifier, was equally impressive against 2018 champion Wozniacki, who flew through the first set and was a break up in the second.
Undeterred, Timofeeva, showing impressive power, hauled herself back into the contest and ultimately prevailed 1-6, 6-4, 6-1.
"It was an honour to play here against Caroline today. I'm just beyond happy and couldn't ask for more," she said after setting up a testing meeting next with 10th seed Beatriz Haddad Maia.
With play on outside courts starting more than three hours late due to rain, Sinner was one of the fortunate few to be scheduled in one of the three main arenas, which all have roofs.
The Italian wasted no time dispatching Dutch qualifier Jesper de Jong for the loss of just six games on Margaret Court Arena.
US Open champion Gauff was equally untroubled in sweeping past fellow American Caroline Dolehide 7-6 (7/2), 6-2, while Rublev and Krejcikova had easy straight-sets wins.
