Madrid, May 4 (IANS): Former doubles partners Elise Mertens and Aryna Sabalenka took contrasting paths to a showdown in the quarter-finals of the Madrid Open here on Tuesday.

The 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, while Sabalenka needed only 52 minutes to power past Jessica Pegula 6-1, 6-2.

Initially, Mertens failed to gain much traction with her tactical approach and Halep was rarely troubled by her changes of spin and depth.

But Halep too was error-prone on Tuesday, and her loss of a break in the first set foreshadowed the failure to press home leads that would ultimately cost her the match.

In the second set, Halep 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 committed 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. Two winners from that wing got 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 up serving at 5-6. With Mertens giving nothing away, another sequence of unforced errors flowed from Halep's racquet to take her total to 48.

Mertens, who entered the quarter-finals here for the first time, will face another Madrid last-eight debutante, No.5 seed Sabalenka.

Sabalenka was imperious against Pegula, striking 25 winners to 13 unforced errors.