Smiling assassin Azarenka downs Muchova to reach last eight

NEW YORK (Reuters) - Victoria Azarenka had a huge smile on her face after reaching her first Grand Slam quarter-final in more than four years by beating Czech Karolina Muchova 5-7 6-1 6-4 in the fourth round of the U.S. Open on Monday.

The former world number one had to rally from a set down on Louis Armstrong Stadium to set up a last eight date with Elise Mertens, her first at that stage of a major since she lost to Angelique Kerber at the 2016 Australian Open.

The 31-year-old, twice a champion at Melbourne Park and twice a finalist at Flushing Meadows, has struggled to reach the heights she enjoyed before she took time out to have her son, Leo.

Azarenka won her first title in four years at the Western & Southern Open in the lead up to the U.S. Open, however, and will head into the quarter-finals with a 9-2 record this year.

"It's been a while," she told ESPN. "It's a quite challenging time in the world right now so to have this opportunity to enjoy myself and do what I love to do with a smile on my face, I'm so grateful."

Muchova, the 20th seed, broke Azarenka's serve three times in the opening set but looked increasingly hampered by a problem with her upper left thigh as the unseeded Belarusian whipped through the second set to even up the contest.

The Czech took a lengthy medical timeout and returned heavily strapped for a decider in which she showed glimpses of the all-court game that has marked her out as someone with a big future.

Azarenka moved her around the court as much as she could, however, and sealed the victory with a vintage forehand winner that just kissed the right-hand corner of the court.

With Serena Williams and Tsvetana Pironkova already through, Azarenka is one of three mothers in the last eight.

"That's remarkable, I'm so proud of the ladies," Azarenka said. "I hope it's inspiring others to keep going for their dreams and not just define themselves as mothers."

(Reporting by Nick Mulvenney in Sydney; Editing by Peter Rutherford)