NEW YORK, Aug. 31 (Xinhua) -- Defending champion and third-seeded American Sloane Stephens beat two-time Australian Open winner Victoria Azarenka of Belarus 6-3, 6-4 to book a women's singles top 16 berth of the U.S. Open here on Friday.

The 25-year-old Stephens, unseeded in her first Grand Slam title run here last year, is now the highest-seeded woman left in the draw after top seed Simona Halep and second-seeded Caroline Wozniacki crashed out in the first and second rounds respectively.

Before the start of her U.S. Open title defense, Stephens said that this would be "a completely new tournament this year."

Azarenka, 29, who missed the 2017 U.S. Open due to her child custody case, played in the 2018 U.S. Open on a wild card invitation.

Today's score was not reflective of the intensity, drama and high level of play of the one hour 46 minute tussle between the two familiar rivals, who were meeting for the sixth time in their careers. Stephens has won all the three meetings in 2018 to tie the career record at 3-3.

During the duel, both Stephens and Azarenka pounded groundstrokes and hammered serves with unrelenting ferocity and accuracy.

An early break gave Stephens a lead in the opening set, and she put it away by breaking Azarenka at love in the ninth game.

But the Belarusian's big shots stretched Stephens' defensive skills to the limit in the second set as the American's early break for a 3-1 lead was followed by an exchange of three more breaks before Stephens closed it out by breaking Azarenka in the final game.

"I competed. When things got tough, I just hung in there, battled as hard as I could, ran down every ball," said Stephens.

In a historic footnote to this year's tournament, the match was the first to be played under the Arthur Ashe Stadium retractable roof as it was closed late in the final set after a brief drizzle. The slight interruption seemed to lift Stephens, who had just dropped serve to fanll behind 3-4.

Stephens said she benefitted from the break, during which she changed clothes and came back feeling refreshed.

"I think that helped me a lot," said Stephens, who won the last three games in the cool atmosphere of the closed stadium.

Stephens will face 15th-ranked Belgian Elise Mertens for a quarter-final berth. Mertens shrugged off a late challenge to beat Czech veteran Barbora Strycova 6-3, 7-6 (7/4).

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