Defending champion Sloane Stephens beat two-time Grand Slam champion Victoria Azarenka 6-3 6-4 to reach the fourth round of the US Open in New York.

The American third seed broke twice in the first set and three times in the second at the Arthur Ashe Stadium.

Both players battled hard in the second set and Azarenka came from 3-1 down to lead 4-3 before the roof was closed.

The 25-year-old French Open finalist will face Belgian 15th seed Elise Mertens in the next round.

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“I just hung in there and battled as hard as I could to keep fighting. I was playing a former Grand Slam champion,” Stephens said. “She raised her level so I just had to stay in it.

“It felt like a night match with the lights on. I have never played under this roof before but it is super cool. It was unlucky for her but I was thankful for the break.”

It was Belarusian Azarenka’s first appearance at the US Open for three years, having missed the 2016 edition while pregnant and last year’s tournament because of a custody battle.

And the 29-year-old showed glimpses of the type of tennis that took her to the top of the world rankings in 2012 but was let down by 27 unforced errors.

Azarenka, runner-up here in 2012 and 2013, believes she can still add to her Grand Slam titles if she can put in a full off-season of training, which she has not managed in recent times.

“Trust me, I wouldn’t be sitting here if I didn’t believe that. I would be home doing a bunch of different things and being successful at those things, but I want to do this,” she said.

“At this stage, it’s a matter of one, two points, and today I just need to cut down my unforced errors and then everything is going to be OK.”

When Stephens takes on Mertens for a place in the quarter-finals, she will be hoping to avoid a repeat of their most recent encounter with the Belgian beating her 7-6 (10-8) 6-2 at the Cincinnati Masters earlier this month.

Mertens booked her place in the fourth round at Flushing Meadows with a 6-3, 7-6 (7-4) victory over Czech 23rd seed Barbora Strycova.

Also through to the next round is Ukrainian seventh seed Elina Svitolina, who overcame China’s Wang Qiang 6-4 6-4.

The 23-year-old will play Latvian 19th seed Anastasija Sevastova, who came from behind to beat Russia’s Ekaterina Makarova 4-6 6-1 6-2.