When an upset happens this late in a tournament, you know it's a shocker. We saw one major upset on Tuesday at the Australian Open, and one almost-upset that would have been even more shocking.

Here's what you need to know about Day 9 of the Australian Open.

Unseeded Madison Keys shocks No. 4 Barbora Krejcikova

Three American women made it to the Aussie Open quarterfinals, and at least one has made it through to the semifinals. Madison Keys, unseeded at the Australian Open and ranked No. 51 in the world, shocked the No. 4 seeded Barbora Krejcikova 6-3, 6-2.

It was clear early on that Keys was playing her best tennis. She won the first game without letting Krejcikova score a single point, and Krejcikova had to battle hard to win the second game. She never quite looked like herself during the game, while Keys was strong and confident from start to finish.

This could be the start of something big for Keys. She had just 11 victories in all of 2021. Her win over Krejcikova on Tuesday was her 11th in January alone. Whatever she's been doing is working, and she'll need to keep doing it on Thursday when she faces world No. 1 Ashleigh Barty.

Rafael Nadal survives upset bid from Denis Shapovalov

It took five sets and a comeback, but Rafael Nadal got it done against 14th-seeded Denis Shapovalov. It was an epic five-set match that lasted over four hours, but Nadal finally managed to win 6-3, 6-4, 4-6, 3-6, 6-3.

Early on, it looked like Nadal was going to power to an easy three-set win. He kept Shapovalov in check through the first set — which included Shapovalov directing an angry tirade at the chair umpire about Nadal getting preferential treatment — but Shapovalov started to battle back in the second set, making Nadal work hard for his points. By the third set, Shapovalov was locked in, and he pretty much demolished Nadal in the fourth set. But that's when we saw vintage Rafa on the court.

Nadal roared back in the fifth set, finally closing out the match after four hours and eight minutes.

We've seen this kind of performance from Nadal before. In fact, we saw it in the last match he played at the 2021 French Open. Up two sets over Novak Djokovic and with the win in sight, Nadal lost the next three sets and was eliminated. Thankfully for him, that bit of history didn't repeat itself, but it was a close call.

Ash Barty rolls past Jessica Pegula

The No. 1 seeded Ashleigh Barty is into the semifinals after beating No. 21 Jessica Pegula, one of the three American women to make it into the quarterfinals.

Barty made quick work of Pegula, beating her 6-2, 6-0 in just 63 minutes, even though she wasn't quite as sharp as she normally is. Despite that, she was still able to toss off shots like this while looking extremely casual.

That's why she's the world No. 1, after all.

Pegula is the second straight American Barty has beaten in the Australian Open. Barty will have a chance to make it three in a row when she faces Keys in the semifinals.

Road ends for Gael Monfils

It had been six years since 35-year-old Gael Monfils had been to the Australian Open quarterfinals, and he put up a fight for the ages to make it farther. Unfortunately, No. 7 Matteo Berrettini put a stop to that in a five-set thriller, beating Monfils 6-4, 6-4, 3-6, 3-6, 6-2.

Monfils had to dig deep numerous times against Berrettini. He was down two sets to zero and on the edge of being eliminated, but managed to find another gear. He roared back in the third and fourth sets, holding Berrettini off and moving around the court like he was a decade younger.

But after tying the game at two sets apiece, one more set was one too many for Monfils. He was drained, and Berrettini easily took the fifth set. He'll face Nadal in his first-ever Australian Open semifinals.