Eight-time winner Novak Djokovic fought through what he had described as a tournament-ending injury to beat Milos Raonic and reach the Australian Open quarter-finals on Sunday.
Naomi Osaka and Serena Williams survived scares to go through but US Open champion Dominic Thiem was a major casualty when he fell to Bulgaria's Grigor Dimitrov.
Simona Halep avenged her crushing French Open defeat by Polish teenager Iga Swiatek to set up a meeting with Williams, while Taiwan's Hsieh Su-wei reached her first Slam quarter-final at the age of 35.
Djokovic, the world number one, had raised the alarm over an abdominal injury after his five-set battle with Taylor Fritz, but he showed few outward signs in his 7-6 (7/4), 4-6, 6-1, 6-4 win.
However, the 17-time Grand Slam winner said he had spent "every single hour" since Friday's win trying to recover, and wasn't sure he would play until he warmed up before the match.
"If it was any other tournament than a Grand Slam I would retire from the tournament, that's for sure," said Djokovic, who becomes just the second player after Roger Federer to win 300 Grand Slam matches.
"But because it's a Grand Slam, I want to give my best alongside my team to try to recover and get on the court."
The defending champion will next meet German sixth seed Alexander Zverev, who beat Serbia's Dusan Lajovic 6-4, 7-6 (7/5), 6-3 as he seeks his first Grand Slam title.
Earlier, Japan's Osaka edged a thriller with fellow major-winner Garbine Muguruza, and Williams came through a physical test against Aryna Sabalenka to maintain her bid for a record-equalling 24th Grand Slam title.
- Osaka, Williams fight on -
In an empty Rod Laver Arena, with fans barred on the second day of a five-day, state-wide coronavirus lockdown, Osaka was on the verge of elimination at 3-5, 15-40 on her serve in the third set.
But the three-time major champion rattled off four points in a row and then twice broke Muguruza's serve to prevail 4-6, 6-4, 7-5.
"I felt like I was intimidated because she was playing so well," said Osaka, after her first meeting with last year's beaten finalist.
"On the stressful points, I had to play within myself."
Osaka now faces an all-Asian quarter-final with Hsieh, who beat Marketa Vondrousova 6-4, 6-2 to reach her first Slam quarter-final in 16 years of trying.
Williams, 39, was also meeting Sabalenka for the first time and she had to weather an onslaught from the Belarusian to reach the last eight 6-4, 2-6, 6-4.
"I knew it wasn't going to be easy, she was teeing off on every shot," said the American, who will play Halep in the quarter-finals.
But a fatigued-looking Thiem, coming off a five-set win over Australia's Nick Kyrgios, slumped in straight sets to Dimitrov who took it 6-4, 6-4, 6-0 in just over two hours.
Dimitrov's victory against Thiem, the third seed and last year's runner-up, sets up a quarter-final with surprise package Aslan Karatsev, who is into the last eight on his Grand Slam debut.
"Whether it's a fairytale or not, it's a match and you've got to be ready," Bulgaria's Dimitrov said of the unheralded Russian.
"I've seen a little bit of his matches, clearly he's a dangerous player."
Karatsev beat Felix Auger-Aliassime in five sets to become the first player in 25 years to reach the quarter-finals on his Grand Slam debut.
The 27-year-old, a virtual unknown before the tournament started, battled back from two sets down to upset the Canadian 3-6, 1-6, 6-3, 6-3, 6-4.
Halep won just three games against Swiatek, 19, at last year's Roland Garros, but she turned the tables with a 3-6, 6-1, 6-4 win.
"I knew she would hit strong, but after the first set I tried to make her run and she started to miss," said the Romanian.