MELBOURNE, Australia -- Serena Williams was one point from victory, one serve from setting up a semifinal rematch against 2018 US Open champion Naomi Osaka. The crowd inside Rod Laver Arena was beginning to thin, as Williams came back from a set and a break down against No. 7 Karolina Pliskova to take a seemingly insurmountable 5-1 lead in the third set. One point and the match was over.

Except it wasn't.

Serving for the match, Williams was called for a foot fault. On her second serve, she ended a seven-shot rally by rolling her ankle and hitting a forehand into the net, pushing the game to deuce. She double-faulted on her next serve and then lost the game on a backhand unforced error. After suffering tough losses in the finals at Wimbledon and the US Open, it seemed improbable Williams would squander another opportunity to capture a record-tying 24th Grand Slam title. Surely, she would settle.

But for the rest of the match, Williams seemed stunned, out of step and unable to recalibrate her game. Once Pliskova realized she had Williams on her heels, she pounced. "I was pushing her. I was more aggressive," Pliskova said after her 6-4, 4-6, 7-5 win. "This is not happening often, maybe once in life. I went for it. I got a chance and I pushed her in the end."

Unbelievably, Williams lost her last 11 points on serve, while Pliskova, a former world No. 1, saved three more match points and won the next five games and the match to earn a spot in the Australian Open semifinals for the first time in her career.

Depending on the angle from which it's viewed, this match will be remembered as either one of the greatest comebacks in Aussie Open history or one of the biggest collapses of Williams' career. In reality, it was both.

"I think it's the best comeback so far in my life," Pliskova said, making a case for the former. "The biggest win against [Serena] is that I believed I could win. That was the victory for me today."

Serena Williams lost the final six games of her quarterfinal match at the Australian Open. WILLIAM WEST/AFP/Getty Images

But let's back up for a moment. To the foot fault. On match point. As the words left the line judge's mouth, the crowd gasped, readying for Williams' reaction. In that moment, was Williams thinking about the infamous US Open incident of 2009, when she was called for a foot fault on her second serve, giving Kim Clijsters double match point? Or her loss against Pliskova in the 2016 US Open semis, when Williams double-faulted on match point? Or maybe she was drawing on a more recent memory, recalling last year's US Open final against Naomi Osaka, when Williams was called for a coaching violation, the first penalty in a string of violations that cost her a point, a game and, arguably, the championship.

Not one to let the public in to her innermost thoughts, it's unlikely Williams will ever discuss what was going through her mind after Wednesday's foot-fault call. It's hard to imagine she wasn't thinking about the improbability of it all. But then again, maybe she remembered she was serving at 5-1 and in possession of the most lethal serve in the game and simply dropped her guard.

Whatever was happening internally, Williams, 37, did not react or question the call. She simply motioned for another ball and continued the match. Outwardly, she kept her cool. But inwardly, she began to unravel.

play 0:15 Fans play rock-paper-scissors over Osaka's towel Naomi Osaka tosses her towel into the stands and two fans determine who keeps the souvenir with a friendly game of rock-paper-scissors.

That's when it all fell apart.

"I did everything I could on those match points," Williams said. "I can't say that I choked. [Pliskova] literally played her best tennis ever on those shots. I've never seen anything like it. Next time I'm up 5-1 against anybody, I need to make sure I play lights out. It's a good learning experience."

But it's not a lesson Williams has studied often. Only twice previously had she squandered a Grand Slam match after being up match point. In fact, the only time Williams had ever lost after being up four match points, as she did Wednesday, came in a 2001 quarterfinal loss to Monica Seles at the Estyle.com tournament in Los Angeles. At this stage of a Slam and against a player like Williams, the chances of a comeback like the one Pliskova orchestrated are so small that the 23-time Grand Slam winner still seemed stunned by the outcome more than an hour after the match ended.

"There's nothing I did wrong on those match points," Williams said. "I don't think it had anything to do with my ankle. I stayed aggressive. She literally hit the lines on some of them. At that point, I'm just trying to think, 'OK, win some points, win this game.' Then I had a couple more match points on her serve. Naturally I thought, all right, here we go, you're going to win one of these. That clearly didn't happen."

play 1:17 Serena suffers foot fault, rolls ankle Late in the third set up 5-1, Serena Williams suffers a foot fault and rolls her ankle on the next rally.

Since returning to tennis from a 15-month maternity leave last March, Williams has made it clear she goes to work every day with one goal in mind: to add to her Slam total and break the all-time record. "She's ready to win all four this year," her coach, Patrick Mouratoglou, told ESPN.com following her fourth-round win here in Melbourne. But after coming achingly close in three of her four attempts in the past 10 months and after experiencing a gut-wrenching loss against Pliskova, one must wonder whether Williams still believes that goal is within her reach.

"The big picture for me is always winning. I'm not going to sit here and lie about that," Williams said. "It hasn't happened yet, but I feel like it's going to happen." When asked where she feels she has the best shot at winning No. 24, Williams said, "Roland Garros. Because it's the next Slam."

Williams lacked that confidence at the start of her quarterfinal match. Instead, Pliskova began the day playing fearlessly and with an aggressive game plan against the seven-time Australian Open champ. But after being broken in the second set, Pliskova dropped her intensity and began to make errors. Williams, on the other hand, started to dictate the pace of the match midway through the second set, playing more aggressively (she was 8-for-10 at the net in the second set) and stealing Pliskova's confidence with each winner (Williams hit 21 winners in the second set to Pliskova's nine). Williams took the second set 6-4 and continued to roll, winning seven of eight games before being broken at 5-1. Serving at 5-2 in the third, Pliskova saw an opening.

"I just felt a chance," Pliskova said of that moment. "For sure it was in her head. I saw a chance and I just took it."

Against Osaka in Thursday's semifinal, Pliskova knows she must do precisely the same.

"I've played few hitters here in the last matches, so I think I'm well-prepared for Naomi," she said. "Obviously, she's trying to make her game from the forehand side. I will just try to do anything possible to maybe put one extra ball back, but on the other hand still be aggressive. Take my chances. I'm sure I'm going to get some. I think anything is possible."

As she learned against Williams on Wednesday, belief is a powerful weapon.