Good friends and now Grand Slam quarterfinal opponents.

After fourth-round victories in Melbourne on Monday, Americans Jessica Pegula and Jennifer Brady will square off for a spot in the final four of the Australian Open. 

The unseeded Pegula, who opened the day on Rod Laver Arena, advanced with a 6-4, 3-6, 6-3 win over No.5 seed Elina Svitolina — her first career win against a Top 10 player. 

Read more: The 100 Club: Jessica Pegula rises through adversity

"I thought I served really well pretty much the whole time," Pegula said after the match. "I think that I kind of got a little tentative in the second set. I played a good game to break her, then I didn't hold serve. That was huge.

"I'm just happy that I reset at the start of the third. I think that's huge. Obviously you know you're going to go three sets eventually. I think the way I kind of switched the momentum and turned it around was really, really important.

"I can't get more confident, it is my best result yet and I'm playing good tennis and today was a hard fought win, so, yeah, I'm feeling pretty good."

Pegula reversed the result of the pair's first meeting — a straight-sets victory for Svitolina in the first round in Abu Dhabi in January — in one hour and 55 minutes.

Sustained power tennis proved to be Pegula's key to victory, as was an ability to reset after inconsistency plagued her in the second set.

"Jen is an awesome person. She's a great person. She was texting me, I'm so happy, I'm so proud. This is awesome... It's an opportunity for both of us. I'm just happy I'm here. She's been playing some good tennis, solidifying herself as a top player. We've all been pushing each other. Why not push each other into a quarterfinal, then one of us be in the semis? That's what we're here to do. We're here to have fun and compete. If I can do it against somebody that I like, that I wouldn't mind if they beat me, hopefully not, but if they did, why not?"

- Jessica Pegula on Jennifer Brady

The American secured the decisive break of serve in the fourth game of the final set, and rode her own service games through to victory.

"It was not a good day for me today," Svitolina said. "I was making way too many errors. Yeah, tough to pick why was like this. A mixture of things. 

"I think she also played quite good today. It was very tough for me to find the rhythm because she was playing good in the first set, not very well in the second. Then she came back.

"I didn't feel very good today on the court. I don't know. It's disappointing because I've been playing very good. I feel it was that kind of day where nothing was going my way."

In all, Pegula nearly doubled Svitolina's total of winners — 31 to 19 — and was successful at net, winning the point on 21 of her 29 trips into the forecourt. 

But also particularly effective, as it turns out, was Pegula's ability to predict the future. 

In the second match on Laver, No.22 seed Brady earned a 6-1, 7-5 win over No.28 seed Donna Vekic in an 1 hour and 34 minutes.

The Croatian entered the match with bandaging on her right knee and later took a medical timeout to add more strapping after holding serve in the first game of the second set.

She battled admirably on in the second set, however, and stay ahead by holding serve until Brady earned the decisive break at 5-5. 

"It was a really tough match," Brady said on-court after the match. "I really had to stay focused in the second set. I was getting a little ahead of myself, but I was able to break and then serve probably my best game there at the end.

"I think in the first set, I came out really strong, wasn't really making many errors. She was giving me a few free points here and there, and I was taking advantage of that. In the second set, she stepped up and started playing better. When she took the medical, I went out, hit some serves, and I was actually starting to feel my serve a lot better."

Brady and Pegula have played once before, as last summer's Western & Southern Open. That match was won by Pegula, 7-6(5), 6-4.

"'m super excited for her making her first Grand Slam quarterfinal. I know the emotions that she's feeling. You feel like you're on cloud nine. She's playing great tennis. We have played before. I think we both know each other so well and I'm really looking forward to it. It will be a lot of fun. I think everyone back home in America will be watching, definitely, and there's guaranteed American in the semifinal."

- Jennifer Brady on Jessica Pegula