Seventeen-time Grand Slam champion Roger Federer said it was a "special moment" to return to the big stage as he passed a first-round test at the Australian Open on Monday.

Federer, back in action after a six-month injury lay-off, downed his one-time junior rival Jurgen Melzer of Austria 7-5, 3-6, 6-2, 6-2 in 2hr 6min on Rod Laver Arena.

The Swiss 17th seed will now play American qualifier Noah Rubin in the second round.

"A special moment walking out on the court. A special moment when you get to match point and when you win the match point," Federer said of his comeback.

"I think those three components make it special when you're coming back.

"It's already great to just be here, trying to play with the guys. I'm happy with the more relaxed attitude and also the success came easier."

Federer, at 35, is bidding to become the second oldest man in the Open era to win a Grand Slam title after Australian Ken Rosewall.

Asked about the constant debate about his eventual retirement, Federer said: "It doesn't surprise me because I've been talking about retirement for seven years.

"Naturally that's the perfect opening for people to ask that question yet again, think to themselves, 'Why more?' I get it. It's fine. I'll keep answering it."

World number one Andy Murray was tested and Stan Wawrinka lost his cool as stars struggled on a hot first day.

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.

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.

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.

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.

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.