Rafael Nadal and Maria Sharapova survived huge scares after being pushed to the limit at the Australian Open on Wednesday, but Roger Federer had an easier workout in the gruelling Melbourne conditions.World number three Nadal battled stomach cramps to come out on top of a mammoth five-setter with American qualifier Tim Smyczek, with the 14-time Grand Slam champion prevailing 6-2, 3-6, 6-7 (2/7), 6-3, 7-5 in a 4hr 12min epic.
The Spaniard is returning from injury and with little match practice and said he found it tough going in the humid night conditions, at one point being sick on court.
"For me after a tough period of time out of competition in the last seven months and just playing a few matches, today was a little bit humid and I felt very tired after the first set and for the whole match," he said.The second seeded Sharapova also struggled, melting down as the afternoon heat was turned up.
She needed to draw on all her experience to save two match points before staging a gritty comeback to beat courageous compatriot Alexandra Panova 6-1, 4-6, 7-5.
World number two Federer also had a wobble, dropping the first set before bouncing back to trounce Italian Simone Bolelli 3-6, 6-3, 6-2, 6-2 and move into the third round.
British sixth seed Andy Murray joined him with a 6-1, 6-3, 6-2 drubbing of local hope Marinko Matosevic.
Other seeds progressing included men's and women's seventh seeds Tomas Berdych and Eugenie Bouchard, and 10th seeds Grigor Dimitrov and Ekaterina Makarova.
World number ones Serena Williams and Novak Djokovic play their second round matches on Thursday.
While the searing 42 Celsius (107.6 Fahrenheit) heat of last year has yet to materialise in Melbourne, temperatures topped 33C and players, including Sharapova and Nadal, draped ice towels around their shoulders at the changeovers.
Sharapova, who can unseat Williams as world number one if she wins the title, wilted badly after winning the opening set, firing a slew of unforced errors that almost had her on an early plane home.
"I didn't actually want to be out here for two-and-a-half hours but that's sometimes the way it goes on days when you're not playing your best -- sometimes it's good enough just to get through," she said.
"I was one point away from being out of this tournament twice today and not playing my best tennis so I'm just happy I was able to win that last point."
Out of the blocks
The five-time Grand Slam champion, red-faced from the heat, trailed 4-1 in the deciding set and looked out for the count, but rallied to save two match points when down 4-5 before hanging on for an unlikely victory.
Federer, chasing his fifth Australian Open title, enjoyed slightly cooler, but still humid conditions, later in the day and steadied after losing the first set to sweep past Bolelli, despite a mystery swollen finger.
"Simone played really well in the first set, I wasn't playing badly, and I had to fight in the middle of the third set and go up a break to enable me to relax a little bit and play a touch better because of less pressure," Federer said.
"He didn't have to lose as much as me, I guess sometimes it can be an advantage and sometimes a disadvantage to be favourite."
Federer, chasing his 18th Grand Slam title, has always made the third round in 16 attempts in Melbourne and has reached the semi-final stage for the last 11 Australian Opens.
Murray, a three-time runner-up, was always in charge against Matosevic, continuing his dominant record against Australians, having now won all 10 previous Tour level meetings with Australian players.
The Scot jumped out of the blocks and wrapped up the opening set in just 21 minutes but had some tighter games in the second and third sets.