MIAMI, FL, USA - Rising Czech Marketa Vondrousova scored another big win on the North American hard courts at the Miami Open on Sunday.

The 19-year-old left-hander upset Doha Qatar Open champion and No.16 seed Elise Mertens to reach the round of 16, 6-4, 7-6(1). 

The victory is Vondrousova's fifth against a seeded player over the past two weeks, having defeated three - Daria Kasatkina, Jelena Ostapenko and Simona Halep - en route to the quarterfinals last week in Indian Wells.

"I feel great on the court. I'm happy with my play and overall, it's such a great tournament. I'm so happy to be through to the fourth round," Vondrousova said.

"This one was tough, because there was a lot of running. Elise is a very good player so it was tough. I'm glad that I made it in the second set and that we didn't play the third set."

The World No.59 never trailed over the course of nearly two hours on court Butch Buchholz, but nearly led big leads slip in each set before righting the ship. 

The Belgian drew even at 4-4 from a 4-1 deficit in the opener, and won three straight games after trailing 3-5 in the second set.

However, a dominant performance from the Czech in the tiebreak saw her win seven straight points to book her spot opposite either No.4 seed Sloane Stephens or Germany's Tatjana Maria in the last 16.

"It's fourth round, so it's going to be tough. My coach will see the match and then we'll talk about it. I mean, they're both very good players...so I'm looking forward to it," Vondrousova said.

"Miami is such a nice place...and I like the new venue so much, so I'm enjoying it."