HOBART, Australia - A quarterfinal clash between two former champions of the Hobart International went the way of a resurgent Heather Watson. 

The British qualifier upset top seed Elise Mertens, 6-7(5), 6-4, 7-5 in three hours and 33 minutes on Thursday as afternoon turned to evening to score her first Top 20 victory in three years.

Mertens, a two-time winner in Tasmania in 2017 and 2018, battled from a break down in the opening set to ultimately put herself on the brink of victory twice in the match.

The Belgian was two games away from victory in the second set, and after a rain delay, was twice a break up in the final set, before Watson pulled off the upset to earn her first-ever victory against Mertens in four meetings.

"I've played Elise before this, and she's beaten me every time. I'm very happy with how I played today, especially under the circumstances," Watson said.

"I thought it was a big test for me against a top player, one I’d never beaten before. I stayed there, fought every point, no matter how far down I was in the score, whether it was 40-0 in a game or 4-2 with a break. I wanted to make sure I gave everything I had today, win or lose."

Watson sets up a semifinal clash with No.3 seed Elena Rybakina of Kazakhstan, who advanced to the final four with a similarly lengthy 6-7(4), 7-6(2), 6-4 win over Aussie wildcard Lizette Cabrera.

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Watson, now ranked World No.101 and a qualifier into this year's main draw, was quick out of the gates, and won the first three games of the match en route to building early leads of 3-0 and 4-1.

However, the top seed responded with a run of three straight games of her own to level the set at 4-4, and there was little to separate the two through to the end of the opener, which was won by the Belgian as she claimed four of the last six points of the tiebreak.

The former Australian Open semifinalist and World No.17 led by a break herself twice in the second set before the former Top 40 Brit rallied to force a decider, with rain showers stopping play three times in the eighth game of the set. 

After two brief interruptions where the players remained on court, a third delay sent them off for about 45 minutes, and Watson forced a decider upon resumption thanks in part to her winning the last four games of the set.

“It didn’t affect me - I expect the rain here in Hobart, I’ve had it so many times before,” she said. “I was very relaxed, actually. I didn’t overthink, I was just relaxing.”

In a final set that saw seven breaks of serve and six marathon multiple-deuce games, Mertens was the first to strike, as she led 2-0 and 3-1 before bouncing back to lead 5-4 from a break down at 4-3.

However, it was the 2015 Hobart champion who found her mettle in the end, as she broke serve for the final time at 5-5 and saved two break points before serving out the match.

"It's been busy, and I feel like I've pretty much been living here at the club, but it's not a bad place to be," Watson said. "I just need to take care of my body now, and get ready for my next match.

"I've played a lot of tennis, and that's great. I'm not complaining at all. I remember this time last year I was struggling to win matches, and I’m glad I’m winning and I’m appreciating every win that I get."

2020 Hobart Highlights: Watson stuns top seed Mertens to progress