RABAT, Morocco - Top seed Elise Mertens is through to the quarterfinals at the Grand Prix Son Altesse Royale La Princesse Lalla Meryem in Rabat after weathering a rollercoaster start against Laura Siegemund, before the German was forced to retire in the third set, 6-7(5), 6-0, 3-1.

“It’s really a double feeling,” Mertens said in her on-court interview. “I played really well in the first, came back in the second and in the third it was a real battle. Of course you want to win, but to win like this is a double feeling. But of course I’m happy to be through in the end.”

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The match started off as a see-saw battle, with Mertens and Siegemund staying toe-to-toe for much of the tightly contested opening set. After splitting six breaks between them, Siegemund saved five set points in a marathon 10-minute game to send them into a tiebreaker. The German surged ahead there, reeling off five points in a row to take a 5-0 lead and weathering a late wobble to close things out 7-5.

Despite seeing five set points slip away, Mertens recovered emphatically - she reeled off two breaks in a row to start the second set with a 4-0 lead, and dodged another three break points from Siegemund to storm through the second-set in a shutout.

She continued relentless in the third set, grabbing another early break to start off 3-1, when Siegemund lunged for a volley and appeared to tweak her left calf. After a medical time out, the German wasn’t able to continue and was forced to retire from the match after a hard-fought two hours and 18 minutes.

The victory sends Mertens into her fourth quarterfinal of the year, and is the Belgian’s 10th consecutive win - a streak that includes lifting the title in Lugano and a pair of Fed Cup wins.

Mertens will take on Sara Errani in her next match after the Italian knocked out Johanna Larsson, 6-3, 6-0 earlier in the day.

“I just see it match by match, every opponent is different,” Mertens said. “If my level is there, and I’m mentally there and physically good, then I know I can go far in this tournament.”