Serena Williams breezed into the Australian Open third round but Petra Kvitova and Bianca Andreescu were the biggest casualties on the third day.
American Williams, who has dropped just five games in her two matches so far, beat Serbia’s Nina Stojanovic 6-3 6-0.
Czech ninth seed Kvitova, who reached the final in 2019, was stunned 6-4 1-6 6-1 by Sorana Cirstea of Romania.
Andreescu, playing in her first major since winning the 2019 US Open, lost 6-3 6-3 to tricky Taiwanese Hsieh Su-wei.
Canadian eighth seed Andreescu was playing in just her second match in 15 months after struggling with a knee injury.
Last year’s finalist Garbine Muguruza continued her good form on the hottest day of the tournament so far with a 6-3 6-1 win over Russian Liudmila Samsonova.
Belarusian seventh seed Aryna Sabalenka also advanced, beating Daria Kasatkina of Russia 7-6 (7-5) 6-3.
Andreescu and Cirstea were among the 72 players confined to their hotel rooms and unable to train on their arrival in Australia as part of a ‘hard’ quarantine imposed after there were positive coronavirus tests on their charter flights.
Andreescu had withdrawn from last week’s warm-up event to protect her Australian Open chances and she moved well in her opening match against Mihaela Buzarnescu.
However, she appeared frustrated and uncomfortable against Hseih, falling to a 4-0 deficit in the first set before mounting a brief comeback.
In 30C temperatures, Andreescu was broken early in the second set and surrendered the match on a double fault.
“I got off the court and I was looking at my coach, and I said, ‘I’m just disappointed that today that was my all,’ Andreescu said.
“I know I gave my all today and to me, I think that was the most disappointing part.”
Hseih, known for her unorthodox shot-making, has beaten a number of top players, including then-world number one Simona Halep at Wimbledon in 2018.
Cirstea had complained before the tournament about the 14-day quarantine, saying the rules changed “overnight” after a passenger on her flight tested positive for Covid-19.
She advanced to the quarter-finals of the Grampians Trophy in the lead-up to the Grand Slam but admitted she had surprised herself against Kvitova.
“I think it’s impressive to come out of 15 days without hitting a ball and competing the way I did today, I wasn’t expecting it,” Cirstea said.