The world number nine looks rejuvenated in Melbourne and, with Andy Murray and Novak Djokovic out, he looks a genuine contender to secure a 15th grand slam triumph.

Watching on in Nadal's box was his girlfriend Xisca Perello, who has never before joined him for his Australian Open campaign. The couple visited Sydney this month after he lost in the quarter-finals in Brisbane.

"The negative thing is I lost in the quarter-final in Brisbane but the good thing is I had chance to visit Sydney," Nadal said in his on-court interview with two-time former Melbourne champion Jim Courier.

"After 10 years, finally my girlfriend gets a wildcard to come here."

😂😂😂

Smooth Rafa... #AusOpen #Nadal pic.twitter.com/RK953jWZaX

— #AusOpen (@AustralianOpen) January 23, 2017

"I hadn't been there before and we enjoyed it. So after 10 years finally my girlfriend gets a wild card to come here."

Asked to elaborate, Nadal said, smiling: "You know... better we stop here."

Nadal lost in Brisbane to Milos Raonic and they will now meet again after the big-serving Canadian had earlier beaten Roberto Bautista Agut 7-6 (8/6) 3-6 6-4 6-1.

The bottom half's other quarter-final will pitch Belgium's David Goffin against Bulgarian Grigor Dimitrov.

Goffin beat Dominic Thiem 5-7 7-6 (7/4) 6-2 6-2 while Dimitrov knocked out Djokovic's conqueror Denis Istomin 2-6 7-6 (7/2) 6-2 6-1.

Nadal ended last season in October to overcome a niggling wrist problem but, after proving his fitness in a five-set thriller with Alexander Zverev, he has now shown his form against the tricky Monfils.

"For me, it means a lot," Nadal said. "To start the season playing quarter-finals in a grand slam again, especially on hard court after a couple of years without being in this round is great news.

"I'm very happy with an important victory against a very good opponent."

Monfils was his usual bag of tricks, with outrageous forehands, volleys through the legs and dummy drop-shots all keeping a packed crowd entertained.

The world number six looked likely to force a decider when he led by a break and 4-3 in the fourth but Nadal came storming back to win three games in a row and the match.

"Sometimes it's tough to play against Gael when you don't know what he's going to do," Nadal said.

"First two sets were a little bit more normal, normal tennis. Then he started to play more aggressive. He start to serve huge and play big shots.

"He combines mistakes with great shots, no? Makes you feel that you don't know if you have to go for the points or if you have to wait a little bit for his mistake.

"So it's not an easy way to play. But I made it. That's the positive thing, I keep fighting with a positive attitude and finally I played the last few games good."

Nadal's loss to Raonic in Brisbane was only his second in eight meetings, but he is under no illusions about the scale of the challenge ahead.

"I need to be very focused with my serve and play aggressive," Nadal said. "If I am not playing aggressive, then I am dead."

Also enjoying a return to form is Dimitrov, the world number 15 long tipped for a big future in the game but struggling to fulfil his potential.

However, the 25-year-old's victory over Istomin earns him a first grand slam quarter-final since 2014 and he believes there is more to come.

"I've learned a lot of new things about myself, I think, starting to work with Dani (Vallverdu), my coach," Dimitrov said.

"We had quite a few big talks. We discussed what we can do better, what we need to work on, what we need to improve, and how we want to be in 2017 - simple as that."