An emotional Petra Kvitova cried "happy tears" after reaching the semi-finals of a grand slam for the first time since the horrific stabbing that almost ended her career.

It is only two years since the Czech was watching the Australian Open on TV wondering whether she would ever play to the same level again after the tendons and nerves of the fingers in her playing hand were damaged in the attack by an intruder at her home.

After beating home hope Ashleigh Barty 6-1 6-4, Kvitova was asked by Jim Courier in an on-court interview whether she had thought she would be back on such a stage.

With tears in her eyes and her voice wavering, Kvitova said: "I didn't really imagine to be back in this great stadium and play with the best. It's great."

"I thought that question will come," she added in her press conference later. "It was kind of a mix of emotions of everything I've been through.

"Sometimes I'm not really recognising anything from the past. But, when Jim asked that, it wasn't really easy for me to see myself being in a semi-final after everything.

"I always wanted to come back and play on the highest level I can, compete with the best, play the grand slams, actually be very deep in the grand slam, which is happening. It just took me a bit to tears, but it was happy tears, for sure.

"I'm calling it my second career. So it's the first semi-final of the second career."

Kvitova has been in ruthless form, not losing more than five games in any of her matches so far to make the last four at a slam for the first time since winning her second Wimbledon title in 2014.

She and Barty, who was the first Australian woman to make it this far in Melbourne for a decade, had contested a very close final in Sydney a week and a half ago but this time Kvitova was simply too good.

Barty recovered from a one-sided opening set to push her opponent in the second and had early chances but was happy to admit afterwards that she had simply been outplayed.

"Petra was outstanding, she really was," said the 22-year-old. "She took it away from me quite early in the match. I have to give all credit to her."

Kvitova and Barty are two of the most well-liked players on tour, and the Australian paid tribute to her conqueror, saying: "She's an amazing human being.

"I think she's beginning to play her best tennis again. We all know what Petra can do. She's a grand slam champion.

"But, most importantly, I think from all of the girls in the locker room, it's amazing just to see her back out here. It wasn't the same when she wasn't here."

Hometown hope Ashleigh Barty missed out Credit: PA

Kvitova might yet end the tournament as both the Australian Open champion and world number one.

Her victory ensured Simona Halep cannot hang onto top spot but Naomi Osaka, Elina Svitolina and Karolina Pliskova, all of whom play on Wednesday, are also in contention.

Kvitova was agonisingly close to reaching number one here seven years ago, but she said: "I don't really care, to be honest. I'm in the tournament, in a slam. I don't think there's any room here (pointing to her head) to think about it. In the next match, playing Danielle in the semi-final, that's what matters right now."

Danielle is unseeded American Danielle Collins, who continued her remarkable run on her debut at the tournament by seeing off Anastasia Pavlyuchenkova 2-6 7-5 6-1.

The 25-year-old was ranked outside the top 100 a year ago and arrived in Melbourne having never won a match in the main draw of a slam.

Collins' relatively late breakthrough can be mostly attributed to her taking the unusual path for a top female singles player of going to college in the United States.

She said: "I think not being a child prodigy certainly humbled me, made me in a way work harder for things.

"This has all been a really incredible experience. This time last year I was playing a challenger (second-tier event) in Newport Beach. But I think I'm really embracing it."