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Flushing Meadows, NY (Sports Network) - World No. 1 and four-time U.S. Open titlist Serena Williams straight-setted fellow American Sloane Stephens in fourth-round action Sunday at the U.S. Open.

The reigning U.S. and French Open champ Williams, a 16-time Grand Slam winner, smashed six aces en route to a 6-4, 6-1 triumph over the 15th-seeded Stephens on the hardcourts at the USTA Billie Jean King National Tennis Center.

"I definitely think it was a high quality match. I don't think either of us had an overwhelming amount of unforced errors," said Williams.

Williams, who defeated Victoria Azarenka in last year's outstanding final in Flushing, will take on Carla Suarez Navarro in the quarterfinals after the 18th-seeded Spaniard upended eighth-seeded Angelique Kerber of Germany by a 4-6, 6-3, 7-6 (7-3) margin.

This year's Wimbledon runner-up was knocked out Sunday, as third-seeded Agnieszka Radwanska was bested by 24th-seeded Russian Ekaterina Makarova, 6-4, 6-4. Radwanska has never reached the quarterfinals at the Open.

Li Na was also a straight-set winner, as the fifth-seeded former French Open champion dominated former world No. 1 Jelena Jankovic, 6-3, 6-0, in just 57 minutes.