Flushing Meadows, NY -- (Sports Network) - Two-time champions Venus Williams and Serena Williams were both straight-set fourth-round winners Monday at the U.S. Open.

The seventh-seeded Venus handled ninth-seeded Agnieszka Radwanska of Poland 6-1, 6-3 at Ashe Stadium. Venus advanced in 1 hour, 24 minutes with the help of five service breaks. The big American moved on despite committing 18 more unforced errors (31-13) than her Polish counterpart.

The former top-ranked Venus is a seven-time major titlist, including the last two Wimbledon championships. She captured the U.S. Open back-to-back in 2000 and 2001 and was the runner-up here in 1997 and 2002.

Venus will battle a familiar face in the quarterfinals as her sister, Serena, the fourth-seed, easily dispatched upstart Frenchwoman Severine Bremond 6-2, 6-2 in just one hour. The former world No. 1 Serena owns eight major titles, including the 1999 and 2002 U.S. Open crowns. She was the 2001 runner-up to Venus and beat her older sister in the '02 finale here.

French Open and Olympic runner-up Dinara Safina was an easy fourth-round winner on Monday.

The sixth-seeded Safina handled German Anna-Lena Groenefeld 7-5, 6-0 on Labor Day at the USTA Billie Jean King National Tennis Center. The red-hot Russian advanced in 1 hour, 15 minutes with the help of five aces and six service breaks, compared to only two breaks for Groenefeld.

Safina reached the finals in six of her previous seven tournaments, including three titles, heading to New York, where she was a quarterfinalist two years ago.

The 22-year-old Safina, the younger sister of men's two-time major champion Marat Safin, lost to current world No. 1 Ana Ivanovic in the French Open finale back in June and gave way to fellow Russian Elena Dementieva in last month's gold-medal match in Beijing.

Safina's quarterfinal opponent will be 16th-seeded Italian Flavia Pennetta, who cruised past 32nd-seeded Frenchwoman Amelie Mauresmo 6-3, 6-0. Pennetta handled Mauresmo in 1 hour, 23 minutes, with the help of a whopping 14 double faults by the Frenchwoman. Mauresmo had her fragile serve broken no less than seven times.

The fading Mauresmo is a former world No. 1 and two-time major champion.

The 26-year-old Pennetta, a two-time titlist on the WTA Tour this season, will now appear in her first-ever Grand Slam quarterfinal.