Roger Federer admits his Australian Open comeback was harder than he expected after the 17-time grand slam champion battled past Jurgen Melzer to reach the second round.

Federer was making his first competitive appearance in more than six months in Melbourne and the Swiss emerged unscathed, beating Melzer 7-5 3-6 6-2 6-2.

After sitting out the second half of 2016 to overcome injuries to his left knee and back, Federer showed signs of rust on Rod Laver Arena but he also improved as the match went on, giving away only four games and not a single break point in the final two sets.

How the 35-year-old's form, and fitness, copes under the pressure of tougher opposition remains to be seen but he should find some leeway again in round two, against American qualifier Noah Rubin.

"I think there were definitely some nerves there," Federer said.

"I was actually fine all day and in the warm-up five minutes before with Jurgen. I felt fine.

"Then I hit four frames in a row. It was like, 'Whew, it's not as easy as I thought it was going to be'.

"I think I struggled for a while to find that groove, that rhythm and everything. Then you remind yourself how many times has it not been easy in the first round.

"Almost every time, except if you get off to a flyer and you keep rolling. But that's not the whole time. So you got to work. I'm happy I was made to work."

Also through to round two is fellow Swiss Stan Wawrinka but the number four seed was pushed all the way by Slovakian Martin Klizan in a five-set thriller.

Wawrinka trailed by a set and then a break at 3-4 in the decider but the reigning US Open champion dug deep to fight another day.

"It was a tough one, for sure," Wawrinka said. "But I'm really happy to get through in five sets.

Federer beat Melzer in four sets (Getty)

"It wasn't my best tennis today but I was fighting, trying to stay in the game, fighting a lot."

Wawrinka is due for a mouthwatering clash with Nick Kyrgios in the last 16 and the Australian also progressed after thrashing Portugal's Gastao Elias 6-1 6-2 6-2.

Kyrgios could not resist a slight dig, however, at Federer, who told Fairfax Media the 21-year-old is still not ready to win a major tournament.

"It doesn't bother me at all," Kyrgios said. "I know he's the greatest of all time.

"I know I just want to do my own thing. I played him once, I beat him once."

Kyrgios' compatriot Bernard Tomic was also in a defiant mood after he strolled past Brazilian Thomaz Bellucci 6-2 6-1 6-4 on Margaret Court Arena.

Tomic has faced questions about his weight and fitness in the build up to this tournament but the 24-year-old believes his performance answered his critics.

Wawrinka could face Kyrios in the last 16 (Getty)

"It was a good match for me. I was working hard when everybody was saying I wasn't ready and not fit," Tomic said.

"Obviously I proved that because the first round was very high-quality tennis for me."

It was another Australian, however, who provided the story of the day as 17-year-old Alex de Minaur saved a fourth-set match point to beat Gerald Melzer 5-7 6-3 2-6 7-6 (7/2) 6-1 on his grand slam debut.

"Words can't describe how I'm feeling right now," said De Minaur, who plays American Sam Querrey in the second round.