MIAMI, FL, USA - Few would have predicted No.21 seed Anett Kontaveit and No.27 seed Hsieh Su-Wei to be the Miami Open's two quarterfinalists at the top of the draw at the start of the tournament - but both have displayed red-hot form, in very contrasting ways, to move into their last eight berths.

Hsieh, one of the tour's premier upset artists over the past two years, scored a sixth career Top 10 win and second victory over a reigning World No.1 in coming from a set and a break down to eliminate Naomi Osaka in the third round, and followed it up by besting Caroline Wozniacki for the first time in five meetings - all the more remarkable considering that the Chinese Taipei player only beat a Top 10 opponent for the first time at the age of 31 (Johanna Konta at Roland Garros 2017).

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Kontaveit, meanwhile, avoided the upset against American teenage prodigy Amanda Anisimova in a high-quality second-round encounter, battled for nearly two-and-a-half hours to edge Ajla Tomljanovic in a third-set tiebreak and then ended Indian Wells champion Bianca Andreescu's run in the fourth round when the Canadian retired with a shoulder injury.

The pair have only played once previously - in the first round of Guangzhou in 2015, which the Estonian won 6-1, 6-3 - but they are nonetheless highly familiar with each other: between 2014 and 2015, they shared a coach, Paul McNamee, and trained together regularly.

"I remember we had some great practices together, and, yeah, we get on really well," reminisced Kontaveit, and Hsieh was quick to return the praise.

"A very, very lovely, nice girl!" gushed the 33-year-old. "She hits the ball different than other girls. Maybe more like Naomi Osaka, because they're both very aggressive."

Neither are currently working with McNamee in Miami: though Hsieh's long-time mentor, whom she credits with turning her doubles career around in 2013, still advises her at major tournaments, her traveling coach is boyfriend Frederic Aniere; while Kontaveit moved on from his tutelage after a year, and is now coached by Nigel Sears.

Read more: 'I just let her be herself' - Hsieh forms perfect match as boyfriend doubles as traveling coach

However, what sticks in the memory or both from their year as sparring partners was that each other's game style was just as extreme in practice. Hsieh recalls being surprised that Kontaveit, whose raw power has been splendidly controlled this week, didn't tone it down - encouraging Hsieh to also turn up the intensity.

"That's only time I practice with a girl I make three ace, I still remember," she laughed. "Because normally my game is not like ace, ace, ace, but that time was just practice, and she was hitting the ball so hard. So I was thinking, okay, it's a practice. So I'm gonna go super hard."

But did Hsieh restrain herself from deploying her full repertore of unconventional shots with nothing at stake? Not a bit of it. "Full on," smiled Kontaveit. "She mixes it up all the time."

Their quarterfinal is the only one this week not to feature a Top 10 player - but Kontaveit's power and Hsieh's trickery are set to make for a riveting contest.