Rafael Nadal Tuesday said his back is still "not great" despite opening his campaign for a record 21st Grand Slam title with a routine straight-sets win.

The world number two pulled out of the ATP Cup last week with the niggle and said ahead of the Australian Open that he was "suffering".

He motored through his first-round clash against valiant Serb Laslo Djere with little drama, racing to a 6-3, 6-4, 6-1, victory under blue skies on Rod Laver Arena, but admitted he was still struggling.

"My back is not perfect, as I said a couple of days ago," said the Spaniard, whose only appearance this year had been an exhibition against Dominic Thiem in Adelaide 12 days ago, where he first experienced stiffness in his back.

"Every day that I'm able to go through, probably there are more chances to be better. That's the thing now. 

"There is always a chance to improve, and that's why I'm here playing and fighting to try to get better and then give myself a chance."

The injury forced him to make changes in his service action against Djere, who didn't have the weapons to trouble the second seed despite Nadal being below par.

"Today it's not great. I needed to change a little bit the motion of my serve," he said. 

"I need to go day-to-day and just try to stay positive. I'm trying to do all the things possible to be ready for compete -- that's what I came here for."

It was his first competitive match since the semi-finals of the ATP Finals in London on November 21.

Nadal raced to a 3-0 first-set lead and wrapped it up before a tougher set two as Djere raised his first serve percentage and got himself in the rallies, with Nadal saving three break points.

But the unrelenting Spaniard was never in jeopardy, dominating off his first serve and outlasting the 56th-ranked Serb on the long rallies.

"I wish him all the best for the rest of the season," Nadal said. "Laslo has a great story. I wish him the best of luck." 

The 2009 champion, who has a history of back problems, is looking to edge ahead of Roger Federer in career Slam titles, with both men having 20 to their name.

But the king of clay has often laboured in vain on Melbourne's hardcourts, reaching five finals but only having the one title, falling in the quarter-finals last year to Dominic Thiem.

Nadal next plays either Serb veteran Viktor Troicki or qualifier Michael Mmoh and is on course to meet Greek rising star and fifth seed Stefanos Tsitsipas in the quarter-finals.