Erstwhile doubles partners Elise Mertens and Aryna Sabalenka took contrasting paths to a showdown in the quarterfinals of the Mutua Madrid Open.

On the Manolo Santana Stadium, No.13 seed Mertens came from a break down in both the second and third sets to upset No.3 seed Simona Halep 4-6, 7-5, 7-5 in two hours and 34 minutes. Meanwhile, over on the Arantxa Sánchez Stadium, Sabalenka needed only 52 minutes to power past Jessica Pegula 6-1, 6-2.

Mertens had lost four of her five previous meetings against Halep, including all three on clay. But her sole victory in the series, a 3-6, 6-4, 6-3 win in the 2018 Doha final, had been a triumph of the Belgian's tenacity. The pattern repeated on Tuesday as she doggedly clung on through lung-busting rallies and knife-edge games.

"I want to be on court as long as possible, especially after losing the first set," said Mertens. "I didn't think too much about the score, I was just thinking about the way I should play to improve myself."

The result is Mertens' sixth career victory over a Top 5 opponent, and marks the first time Halep has lost before the Madrid quarterfinals since 2015, when she fell to Alizé Cornet in the first round.

Elise Mertens' Top 5 wins
2018 Australian Open QF, d. Elina Svitolina 6-4, 6-0
2018 Cincinnati R3, d. Sloane Stephens 7-6(8), 6-2
2019 Doha F, d. Simona Halep 3-6, 6-4, 6-3
2020 US Open R4, d. Sofia Kenin 6-3, 6-3
2021 Gippsland Trophy QF, d. Elina Svitolina 6-3, 5-7, [10-6]
2021 Madrid R3, d. Simona Halep 4-6, 7-5, 7-5

Initially, Mertens failed to find gain much traction with her tactical approach. Halep was rarely troubled by her changes of spin and depth, and the Mertens backhand down the line misfired time and again.

Halep herself was also error-strewn, and her loss of a break lead in the first set foreshadowed the failure to press home leads that would ultimately cost her the match. But with Mertens still trying to find her game, the two-time Madrid champion found some fine forehand angles to close out the opener.

The hotshots continued into the second set for Halep, who recovered from 1-3 down and broke for 4-3 after a brilliant return game. But the Romanian's lack of focus returned on the brink of victory. Serving to stay in the set, she coughed up four straight unforced errors to find herself in a decider.

By now, persistence had paid off for Mertens. Her backhand down the line was finally clicking, and in some style. Two winners from that wing garnered her a break back for 3-3, and another pair saw her capture the Halep serve again for 4-4.

As in the second set, Halep tripped herself up serving down 5-6. With Mertens giving nothing away, another sequence of unforced errors flowed from the Halep racquet to take her total to 48.

"She played really well, and she definitely deserved to win because she was stronger in the end," said Halep. "I cannot say I played bad, but I did some mistakes, important mistakes. Sometimes it's just a little bit and the match is going away."

Mertens, into the quarterfinals here for the first time, will face another Madrid last-eight debutante in No.5 seed Sabalenka. Indeed, the Belarusian had never won a main draw match in the Spanish capital before this year, and this run marks her first WTA 1000 claycourt quarterfinal.

The pair have won five doubles titles together since 2019, including that year's US Open and this year's Australian Open, but parted ways this season in order for Sabalenka to focus on singles.

"She's on fire, she's making a lot of angles and winners," said Mertens of their upcoming tilt. "I'll just try to put as many balls in the court as I can and try to take over a little bit when there's a shorter ball. She played [Ashleigh] Barty last week, and I'm definitely going to watch a little bit of that match, because Barty also likes slice and everything to mix it up."

Sabalenka was imperious against Pegula, striking 25 winners to 13 unforced errors. She would face, and save, just one break point over the match, while breaking her American opponent four times.

"Everything was really good today," Sabalenka said. "She's an aggressive player and she's playing really well right now. I'm really happy that I could stay on this level and put pressure on her so she couldn't do anything."

Mertens comes back to upset Halep in Madrid third round: Highlights

2021 Madrid