Australian Open

The legends last met on a court playing some playing mini tennis with some juniors and discussed playing a charity match.

Instead they may square off in a grand slam decider that many — including themselves — felt was beyond them.

The last time the duo battled in the final at Melbourne Park was in 2009, which the Spanish bull won.

Federer, 35, is returning from six months out following knee surgery while Nadal, 30, had an injury-ravaged 2016.

The ninth-seeded Nadal joined Roger Federer in the last four, keeping alive the prospect of a dream final between the two great rivals who have claimed 31 major titles between them.

Having not reached a major semi-final since his 2014 French Open title, the 30-year-old Nadal produced a match from his halcyon days to fell the third-seeded Canadian in two hours and 44 minutes on Rod Laver Arena.

Seeded ninth, Nadal looked in full control when up two sets to love before Monfils launched a comeback which fell short 6-3, 6-3, 4-6, 6-4.

Nadal next faces third-seeded Canadian Milos Raonic.

It will be the 30-year-old's ninth appearance in the final eight at Melbourne Park, having won the title in 2009.

It is also the former world number one's first grand slam quarter-final showing since 2015.

Nadal took advantage early of Monfils' inconsistent play, but Monfils showed there was more to his game than flamboyance as he made the veteran earn the win.

During his second-round match, Jo-Wilfried Tsonga noticed a ball girl was fighting back tears after being struck by a ball.

The Frenchman stopped play to check on the girl before looping his arm through hers and escorting her off the court.

Twelve months on, Tsonga has revealed the girl, Guiliana, sent him a heartfelt message of thanks.

"Dear Mr Jo-Wilfried Tsonga," the letter reads.

Viviani, 17, hit a ball away in disgust after losing the first set of her round one tie with China's Xin Yu Wang.

The ball apparently hit the ball kid softly in the chest and Viviani immediately apologised for her actions. 

The chair umpire didn't show any leniency though, disqualifying the Italian on the spot. 

Playing only his third tour match since coming back from a knee injury, Federer was expected to be tested by the hard-hitting Czech but he sent the 10th seed packing after a 90-minute masterclass at a floodlit Rod Laver Arena.

The 35-year-old Swiss, seeded 17th, had shown rust in his earlier matches but was in spell-binding touch with 40 winners, wrapping up the match with an imperious cross-court backhand.

Federer will meet fifth seed Kei Nishikori for a place in the quarter-finals.

Barty, ranked 223 in the world, scored a 7-5, 6-1 win over a player ranked 171 spots ahead of her.

"I've got goosebumps. This is amazing ... I'm shaking," the 20-year-old said.

In even better news for the Queenslander, she will face qualifier Mona Barthel for a spot in the round of 16 after the German beat Olympic gold medallist Monica Puig 6-4, 6-4.

Barty returned to tennis early last year after a stint playing professional cricket, with the Australian Open her first major campaign since the 2014 US Open.

A cooler day with patches of rain is forecast for the day, in what should be a relief for players after previously hot conditions.

So who among the international stars and Australian hopefuls is on court today?

Big guns of men's tennis in action

Arguably history's greatest player Roger Federer is the morning's star attraction as he faces off against USA's Noah Rubin on Rod Laver Arena.

The 28-year-old German was starting the defence of a grand slam title for the first time and initially struggled with her serve and the accuracy of her groundstrokes on Rod Laver Arena.

Once she found her range however, Kerber proved more than a match for world number 51 Tsurenko, who was reduced to scrapping to save her serve and the odd pearl of a consolation point off her rasping backhand.

Federer, who has claimed four Australian Open titles himself, believes Kyrgios isn't experienced enough to go the full length.

"To win, he drops, in my humble opinion,'' Federer told Fairfax Media. 

''To do anything, yes, first week no problem, but to go all the way it's a long way, and he's never been into a semis before, so first he has to prove to the world and himself that he can make the semis.

"So for me to start talking about him winning the tournament, Lleyton [Hewitt] never did it, and he was two times world No.1 at the end of the year.