MIAMI, FL, USA - Anett Kontaveit broke new ground at the Miami Open on Saturday night with an epic victory.

In a match that fell just short of two-and-a-half hours, the No.21 seed edged unseeded Aussie Ajla Tomljanovic in three sets, culminating in a tiebreak, 7-6(3), 2-6, 7-6(4) to reach the round of 16 for the first time. 

In her fifth career main draw appearance in Miami, Kontaveit sealed a spot in the fourth round of any of the four Premier Mandatory events for the first time with the win.

"It was a really tough, really close match all along, and I was just trying to stay really tough in the tiebreak, playing as well as I can," Kontaveit said after the match. 

"I'm definitely very happy. I didn't have any points to defend from last year. I feel like I'm playing better, improving a little bit."

Having needed three sets to defeat American teenager Amanda Anisimova in the first round, Kontaveit spoiled Tomljanovic's attempt to defeat a seeded player for the second straight match in Miami.

The World No.40 had upset No.9 seed Aryna Sabalenka in the secound round, and broke Kontaveit twice at the end of a dramatic final set when the Estonian served for the match at both 5-4 and 6-5. 

Nonetheless, the 2018 Wuhan finalist steadied herself to begin the deciding tiebreak and won the first three points, eventually seeing a 5-2 lead through to victory just after 11 p.m. local time.

"It's tough, but the key for me was to just try and keep doing my thing and stay tough," the Estonian added.

"I was trying to get back as many balls as I can, and make her play as many balls as I can."

In a big-hitting battle, Kontaveit led by a break to begin the match, but ultimately came from a break behind twice to seal the opening set in a tiebreak - which included breaking Tomljanovic to 15 when she served for a one-set lead.

After seeing the Aussie catch fire with her forehand and win five of the last six games to force a decider in their first-ever meeting, Kontaveit nonetheless managed to stay ahead for much of the decider. 

The first break of the final set went the No.21 seed's way, as she sealed her third break point chance from a 0-40 lead to move ahead 3-1, and she stayed ahead until Tomljanovic staved off defeat twice in the set's late stages.

"She played really well, and I thought she did a lot of things really well," Kontaveit said. "My serve was letting me down a little bit, when I was serving for it twice before. 

"I really just had to tough it out and I think that's what it was."

Up next, Kontaveit face either Indian Wells champion Bianca Andreescu or No.8 seed Angelique Kerber, who just took the court as she was wrapping up for their own late-night, third round match in Miami.

"I haven't played Andreescu before, and she's had an amazing run. Kerber is such a great player as everybody knows anyway, so it's going to be so tough either way," Kontaveit said. 

"They were in the final just last week, so I just have to think about recovering right now and enjoy the win for now."