Melbourne - Elise Mertens said it was "amazing" to become the first Belgian to reach the quarter-finals of the Australian Open since Kim Clijsters in 2012.
The unseeded world number 37 edged Petra Martic 7-6 (7/4), 7-5 after a rollercoaster baseline war on Margaret Court Arena.
In-form Mertens is making her Australian Open main draw debut and is on a nine-match win streak after making a successful title defence in a warm-up event in Hobart just over a week ago.
"I've had an amazingly good start to the year," she said after having to grind out a victory in an hour and 59 minutes.
The win over her 81st-ranked opponent elevated the 22-year-old to a Grand Slam quarter-final for the first time in her career.
"It's amazing to be here, I have no words to describe this feeling," said Mertens, who began 2017 ranked 120th and has been one of the fastest movers in the WTA rankings, ending the year 35th.
The Belgian was made to work hard for the win by the never-say-die, big-serving Croat.
Mertens, who for the past three years has trained at Clijsters' academy in Belgium with coach Robbe Ceyssens, led 4-0 in the first set before being taken to a tiebreak.
Serving for the match in the second set she was broken again before finally passing the winning post at the second time of asking.
Mertens is in uncharted territory in her fifth appearance in the main draw of a Grand Slam.
She has only once before got past the opening round in a major, at Roland Garros last year when she reached the third round.
"I worked really hard to get here and this is really a bonus. A quarter-final is not bad but I would like to do even better," she said.
She could next face another in-form player, fourth seed Elina Svitolina who won the Brisbane International warm-up.
The Ukrainian will first need to get through a late match against qualifier Denisa Allertova.
Results from the Australian Open on Sunday:
Marin Cilic (CRO x6) bt Pablo Carreno-Busta (ESP x10) 6-7 (2/7), 6-3, 7-6 (7/0), 7-6 (7/3)
Rafael Nadal (ESP x1) bt Diego Schwartzman (ARG x24) 6-3, 6-7 (4/7), 6-3, 6-3
Kyle Edmund (GBR) bt Andreas Seppi (ITA) 6-7 (4/7), 7-5, 6-2, 6-3
Carla Suarez Navarro (ESP) bt Anett Kontaveit (EST x32) 4-6, 6-4, 8-6
Caroline Wozniacki (DEN x2) bt Magdalena Rybarikova (SVK x19) 6-3, 6-0
Elise Mertens (BEL) bt Petra Martic (CRO) 7-6 (7/5), 7-5