Live: Australian Open, day nine at Melbourne Park

Welcome to our live coverage of day nine of the Australian Open.

Aussie Ash Barty defeated Petra Kvitova to move into the semi-finals while Roger Federer and Novak Djokovic are both in action on Rod Laver Arena today.

Nick Kyrgios lost to Rafael Nadal in the fourth round last night but is back to partner American teenager Amanda Anisimova in the second round of mixed doubles on Margaret Court Arena.

ROD LAVER ARENA

Sofia Kenin (US) [14] defeated Ons Jabeur (Tun) 6-4 6-4

Ash Barty (Aus) [1] defeated Petra Kvitova (Cze) [7] 7-6 6-2

Roger Federer (Sui) [3] defeated Tennys Sandgren (US) 6-3, 2-6, 2-6, 7-6, 6-3

From 9pm: Milos Raonic (Can) [32] vs Novak Djokovic (Ser) [2]

Roger Federer pulls off miracle comeback

Roger Federer was down for the count. His body looked like it had given up on him and Tennys Sandgren had all the momentum in the world.

Down two sets to one the Swiss master looked defeated. Things weren't falling his way and there was an eerie silence around Rod Laver Arena.

But you should never count out the heart of a champion and Federer showed why as he rallied to win the match 6-3, 2-6, 2-6, 7-6, 6-3 and keep his Australian Open hopes alive.

Sandgren will be left to rue what could have been as he squandered seven match points to Federer.

"Gotta get lucky sometimes … I think I got incredibly lucky today … As the match went on I started to feel better again and all the pressure went away," Federer said after the match.

"I don't deserve this one, but I'm standing here, and I'm obviously very, very happy."

Federer came out looking unstoppable as he claimed the first set with ease before Sandgren flipped the script and stole the momentum.

As he took complete control in the third set the frustrations got the better of Federer and he unloaded after another missed shot.

After dropping the game, Federer called for the trainer before taking a medical timeout. He came back out onto the court but looked hampered as he struggled to move around the court and Jim Courier knew something was wrong.

"Houston we have a major problem here with Roger Federer," Courier said.

"Whether it's his back or something else, he didn't bend at all during that serve."

Somehow after looking like he simply couldn't move freely around the court, Federer's body sprung back to life and he slowly began turning the momentum back in his favour.

In the fourth set he forced the contest into a tiebreak and had to dive into his bag of tricks as he saved match point after match point before sending it into the deciding set.

From there it was clear the tennis legend had everything behind him as Sandgren looked powerless to stop the momentum.

The miraculous performance puts him through to another semifinal.

From down and out to in control

Roger Federer simply isn't human.

During the third set the Swiss master was simply out on his feet, he couldn't produce any power off his serve and he struggled moving around the court.

Now he's just one game away from advancing into the next round and is looking like the Federer we saw to start the match.

Seven match points saved - we're off a fifth set

After an incredible display at the back end of the fourth set, Roger Federer and Tennys Sandgren are heading to a fifth set.

The world number 100 looked to have it won, having multiple chances to cause the upset, but Federer fought back every time and takes momentum into the deciding frame.

Federer showing signs of fight

He's clearly not remotely close to 100 per cent, but Roger Federer isn't going down with a fight as he forced the match into a fourth set tiebreak.

Tennys Sandgren jumped ahead and held three match points, but Federer wouldn't be denied and saved every single one as he rallied to force the match into a fifth set.

"How did he get out of that," Todd Woodbridge proclaimed.

Sandgren takes third set

The curtain looks to be drawing to a close on Roger Federer's 2020 Australian Open as he falls behind two sets to one against American Tennys Sandgren.

Sandgren has amplified the pressure on the Swiss master by claiming a drama filled third set 6-2.

Fed unloads, cops obscenity warning

Roger Federer's feathers have clearly been rattled in his contest against Tennys Sandgren with the Swiss master being slapped with a verbal obscenity warning.

Holding two break points, Federer slammed a backhand into the net before he let rip verbally.

While his outburst isn't entirely clear, the F bomb can be picked up in the audio.

Moments after a line judge notified the chair umpire who handed Federer the verbal obscenity warning only for the Swiss legend to approach the line judge before taking up the matter with the chair umpire.

After dropping the game, Federer called for the trainer before taking a medical timeout.

He came back out onto the court but looked hampered as he struggled to move around the court and Jim Courier knew something was wrong.

"Houston we have a major problem here with Roger Federer," Courier said.

"Whether it's his back or something else, he didn't bend at all during that serve."

Fed shocked: 'They can't believe what they're seeing'

Roger Federer showed glimpses of his classy best as he won the first set of his quarter-final against Tennys Sandgren 6-3.

Sandgren stayed with the Swiss maestro early in the set but Federer pulled away to grab the early lead.

The momentum shifted dramatically in the second set though as Sandgren crashed Federer's party. The unseeded American won the set 6-2 to bring the match back on equal footing.

Federer's level dropped significantly, making 15 unforced errors in the second set compared to just seven in the first.

Sandgren won the first game of the third set, prompting former Aussie star Sam Groth to reflect on how shocked the centre court crowd was.

"They can't quite believe what they're seeing," Groth said in commentary for Channel 9 as he referenced the hush that had fallen around Rod Laver Arena. "Roger's flat.

"I don't think anyone knows how to react right now."

Barty gets her sweet revenge

Ash Barty has taken her revenge on Petra Kvitova, defeating seventh seed 7-6 (8-6) 6-2 to advance to the Australian Open semi-finals.

After being knocked out of her home grand slam by the Czech 12 months ago, Barty wasn't going to let the same thing happen this time around as she sent the crowd at Rod Laver Arena wild with her straight sets victory.

The win is Barty's first over a top-10 opponent at a major and sees her become the first homegrown talent to qualify for the semi-finals of the Australian Open since Lleyton Hewitt in 2005.

"It's been absolutely incredible. I knew I had to be at my absolute best against Petra – that first set was so crucial," Barty said.

"I just wanted to try to make the most of it and (it was) really good to get a quick start in the second."

The first set was gruelling slugfest that lasted 69 entertaining minutes and while Barty looked on the back foot at times, she showed enough grit – and Kvitova made enough errors – to take the early advantage.

The first set may have been an arm wrestle but Barty was steamrolling her Czech opponent in the second, getting a double break and charging to a 4-0 lead in just 20 minutes.

Kvitova finally got on the board, holding serve for the first time in the set on her third attempt to make it 1-4 before getting a break back.

The left-hander couldn't consolidate though and Barty broke once again to go 5-2 up.

The world No. 1 had some nervy moments while serving for the match but held her nerve to seal her progression through to the final four.

Kvitova would have been hoping for a very different story when she started by unleashing some ferocious hitting to break Barty to love in the third game of the match, sealing it with a blistering cross-court forehand winner.

"This is some ominous hitting from Kvitova here in the early stages of this match," Channel 9 commentator Jim Courier said.

Barty wasn't on the back foot for long though, breaking to love herself the very next game to make it 2-2 before holding easily for 3-2 in the opening set.

Kvitova then came up clutch on serve to save a break point and level the set up at 3-3 before crunching some more heavy groundstrokes and giving herself two break points.

But Barty stuck solid and forced things to deuce. At deuce number four, the Aussie unfurled a magnificent forehand winner down the line after Kvitova monstered a backhand that by rights should have had the hometown hero beat.

"That is insane how good that is!" Courier said. "Fending off the firepower of the Czech. She somehow finds this shot right over the high part of the net …That was awesome."

The slugfest continued and Barty finally held serve after going to deuce seven times in the same game, taking a 4-3 lead before two more holds made it 5-4 in the top seed's favour.

Neither player could break free and the set went to a tiebreak. Barty won an epic 22-shot rally to get the breaker back on her terms then saved a set point to make it 6-6 at another change of ends.

The Australian kept her cool and claimed the tiebreak 8-6 to grab a vital lead, before enjoying a much easier time of it in the second set.