World number one Andy Murray was tested and Stan Wawrinka lost his cool as stars struggled on a hot first day at the Australian Open on Monday.
Roger Federer and Angelique Kerber, the defending women's champion, both dropped sets on a day which threatened a host of upsets at Melbourne Park.
In the event, fourth seed Simona Halep was the only major casualty as soaring afternoon temperatures gave way to a cool evening at Melbourne Park.
Australia's often hot-headed Nick Kyrgios was uncharacteristically calm as he thumped Gastao Elias 6-1, 6-2, 6-2, despite a nosebleed which halted him mid-match.
But Wawrinka drew gasps as he smashed a volley straight at a static Martin Klizan deep in the fifth set, en route to a difficult 4-6, 6-4, 7-5, 4-6, 6-4 win.
"I stopped playing. He could play anywhere, the whole court," a perplexed Klizan said. "He could play anywhere on the court and he hit me."
As afternoon temperatures topped 32 Celsius (90 Fahrenheit), Murray came through a two hours, 47 minutes test against Ukraine's Illya Marchenko 7-5, 7-6 (7/5), 6-2.
He fared better than Kei Nishikori and Marin Cilic, who were both taken to five sets, while Federer was extended 7-5, 3-6, 6-2, 6-2 by Austria's Jurgen Melzer.
"Any match is a good match," said the 35-year-old Federer, who missed the second half of 2016 with a knee injury.
"Even if I would have lost today, it would have been good because I'm back on the court."
Kerber, defending a Grand Slam title for the first time, laboured past Ukraine's Lesia Tsurenko 6-2, 5-7, 6-2 in an unconvincing performance.
The German was an early loser at both of her warm-up tournaments and she took time to settle against Tsurenko. But she also started slowly last year, when she had to save a match point against Misaki Doi in the first round.
"The first round is always tough. I'm just trying to enjoy it and I'm so so happy to be in the second round," Kerber said.
Halep is rated as one of the fastest players on the circuit but her usual zip was glaringly absent as she went down 6-3, 6-1 to American rising star Shelby Rogers.
Afterwards she said a knee injury had slowed her down as she succumbed in the first round for the second consecutive year.
Venus Williams, the oldest player in the women's draw at 36, also found herself with a fight on her hands but she came through 7-6 (7/5), 7-5 against 22-year-old Kateryna Kozlova.
"Girl, I don't know," she told on-court interviewer Samantha Smith, who pointed out that Williams turned professional in 1994, the year that Kozlova was born.
"I know how to play tennis. I like to think I'm good at this. She hasn't had the years that I've had yet, the grey hairs and I'm dying, I guess. The wrinkles that I'm hiding... you're trying to make me feel old!"
Japan's Nishikori, who says he is primed to win his first major title this year, came through a five-set battle with Russia's Andrey Kuznetsov 5-7, 6-1, 6-4, 6-7 (6/8), 6-2.
Cilic, the 2014 US Open winner, also needed five as he came from two sets down to dispatch Jerzy Janowicz 4-6, 4-6, 6-2, 6-2, 6-3.
China's Zhang Shuai, who made a surprise run to last year's quarter-finals, had an easier time with a 6-0, 6-3 win over Aliaksandra Sasnovich of Belarus.
Olympic champion Monica Puig stormed through 6-0, 6-1 against Patricia Tig before Canada's Eugenie Bouchard continued her recent resurgence with a 6-0, 6-4 win over Louisa Chirico.