MELBOURNE, Australia - No.7 seed Karolina Pliskova saved four match points and pulled off an improbable comeback to complete the 2019 Australian Open semifinal line-up on Wednesday, rallying from 5-1 down to defeat 23-time Grand Slam champion Serena Williams, 6-4, 4-6, 7-5, and book a semifinal clash with reigning US Open champion Naomi Osaka.

"Normally I'm having lot of comebacks, but maybe like from being a set down or a break down, so nothing like this," Pliskova said after the match. "I think it's going to be the best comeback ever so far in my life."

Read more: 'She played lights out on match point' - Serena praises Pliskova

Pliskova looked down and out when Williams suddenly appeared to tweak her ankle a game from victory. That moment proved pivotal as the Brisbane International champion turned the tables and maintained her perfect start to 2019 - winning her 10th straight match - after two hours and 10 minutes on Rod Laver Arena.

"Before the match, this time I really believed, like, it's still a player, I know she's the greatest ever, but I don't want to put her somewhere where I would not have a chance to beat her. I beat her before. She was No.1 at that time. I knew I'm going to have my chances. But I had to play well."

The pair last played in the US Open quarterfinals last summer, and the match began similarly as Pliskova broke first and held four chances for a 4-1 lead.

Though the American kept things close, Pliskova showed improved poise and movement to serve out the opening set and secure the first break of the second.

It was up to the 23-time Grand Slam champion to rely on her fighting spirit and pull off one of her signature comeback performances, which began as she immediately broke back at love.

"Suddenly I lost somehow momentum. I was not playing the same game what I was playing, which was working, till the set and break. I was pushing her. I was more aggressive.

"I stopped little bit. Obviously she took her chance. That's what happened. She just went for it. She went for her serves. She went for the returns. She was just putting pressure on me. I was more passive."

Saving multiple game points as Pliskova served to stay in the set, Serena struck some fearless returns to break again and level the match.

Read more: Conchita Martinez - 'Nadal is a role model' for Pliskova

Shrugging off a 0-30 deficit to kick off the decider, the American raced towards victory in vintage style, taking a commanding double-break lead behind a flurry of winners - 54 total in the match.

It was then that, just short of the finish line, Serena seemingly tweaked her ankle. Still playing solid tennis, Pliskova responded in remarkable fashion, stepping up to save a match point at 5-1. Looking distracted and hampered in her movement, the former No.1 nonetheless engineered three more match points at 5-4, but Pliskova wouldn't be denied she rolled through the final six games of the match.

Refusing to blame injury after the match, Serena said, "There's nothing I did wrong on those match points. I didn't do anything wrong. I stayed aggressive. She just literally hit the lines on some of them. One she hit an ace, unreturnable serve. I literally did everything I could on those match points. 

"I can't say that I choked on those match points. She literally played her best tennis ever on those shots."

Speaking from the ESPN desk, Rennae Stubbs - one half of Pliskova's coaching team alongside Conchita Martinez - said, "I'm so proud of the fact Karolina didn't give up. We know that Serena twisted her ankle and there was the footfault on match point but she continued to play aggressive.

"I know her so well, the fact that she was crying at the end shows how much this meant to her. Karolina is now 15-0 when she wins the first set at the Australian Open."

In all, the 2016 US Open finalist struck 32 winners to just 15 unforced errors. The battle between the game's biggest servers unsurprisingly game down to break point opportunities, with Serena converting four of eight while the Czech took five of 11.

"I feel I'm not doing very stupid mistakes. I'm just taking care of the balls which are in the middle of the court, around the service line.

"I think it has something to do with my legs, of course. I'm more ready for these short balls. We've been practicing that a lot, because sometimes after a great serve, I was just sleeping, not getting ready for the ball, not getting there enough on time."

Standing between the Czech and a second career Grand Slam final is Osaka, who knocked out reigning BNP Paribas WTA Finals Singapore presented by SC Global champion and No.6 seed Elina Svitolina earlier in the day. Osaka has won both of her previous meetings with the former World No.1, most notably at the US Open where the Japanese youngster captured her first major title.

"I've played a few hitters here in the last matches, obviously Giorgi, now Serena. I think I'm well prepared for Naomi. Obviously she's trying to make her game from the forehand side.

"I'll just try to do anything possible just to maybe put one extra ball back, but on the other hand still be aggressive, and taking my chances. I'm sure I'm going to get some. Of course, maybe she, too. But I think anything is possible. She is in the first semifinal here, too.

"Obviously she played well in New York. She didn't have that many experiences in this tournament as maybe Serena. So it's still pretty new for both of us. Obviously it's my first semifinal, too.

"I think if I play well, well, there's going to be a chance. If I play well, I think for sure there is a chance."