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.

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.

Two more fourth-round matchups are slated for Sunday, with third-seeded 2012 Wimbledon runner-up Agnieszka Radwanska battling 24th-seeded Russian Ekaterina Makarova, while fifth-seeded former French Open champion and two-time Australian Open runner-up Li Na takes on ninth-seeded former world No. 1 Jelena Jankovic.