NEW YORK — Finished with a mostly matter-of-fact victory in the U.S. Open's third round yesterday, defending champion Serena Williams switched into a yellow T-shirt with this bit of wisdom in blue script: "Can't spell dynasty without nasty."

Asked about it, Williams took the chance to push a sponsor's new clothing line. Moments later, she found herself discussing her autobiography, though she refrained from shouting: "Available at a bookstore near you!"

About the only thing Williams doesn't need to sell anyone on is the quality of her tennis, particularly at Grand Slam tournaments. By beating serve-and-volleying Spaniard Maria Jose Martinez Sanchez 6-3, 7-5 yesterday, Williams improved to 28-1 over the past five majors.

The secret to her success?

"You can never underestimate anyone. Some people, some days, they have great days," said the No. 2-seeded Williams, whose sister Venus also won yesterday. "I just go and look at every opponent as the best player in the world."

Perhaps some of her colleagues should take that approach, as well.

This has been a topsy-turvy U.S. Open for the women: No. 8 Victoria Azarenka's 4-6, 6-2, 6-2 loss to No. 26 Francesca Schiavone yesterday came a day after No. 4 Elena Dementieva and No. 5 Jelena Jankovic were upset. All told, 11 of the 20 highest-seeded women are gone, and the third round is only halfway done.

"Now there is not a lot of difference between players," Sanchez Martinez said. "I think anyone can win any match."

No. 3 Venus Williams, the 2000-01 Open champion, avoided adding to the list of stunning results when she got past 46th-ranked Magdalena Rybarikova of Slovakia 6-2, 7-5. Next up for the elder Williams: a fourth-round match against Kim Clijsters, who recently came out of retirement and is playing in the tournament for the first time since winning it in 2005.

"She was a great champion," Venus Williams said. "She still is."

No. 1 Dinara Safina made it to today's third round, but barely. She needed more than 4 1/2 hours to get through two three-set victories this week.

The best men have faced no such problems.

Yesterday's winners included No. 2 Andy Murray, No. 6 Juan Martin del Potro, No. 7 Jo-Wilfried Tsonga and No. 9 Gilles Simon. None of the top 10 men had dropped a single set — much less lost — until 2008 runner-up Murray's little slip in his 6-2, 3-6, 6-0, 6-2 win over 87th-ranked Paul Capdeville of Chile.

Capdeville and Murray know each other from way back, when both trained in Spain. Capdeville has seen Murray develop from a kid who would lose focus to a major title contender.

"Five years ago, he was a little young," Capdeville said. "Now he's a man."

Murray now faces Taylor Dent, a wild-card entry from Newport Beach, Calif., who used to be ranked 21st but missed two years after back surgery and now is 195th. Dent's impressive comeback continued yesterday with a stirring 6-4, 5-7, 6-7 (1), 7-5, 7-6 (9) victory over 119th-ranked Ivan Navarro of Spain.

They played for more than 4 hours before Dent finally prevailed. He grabbed the chair umpire's microphone to thank the raucous, partisan crowd for its support, then took a lap around the Grandstand arena, slapping palms with fans.

Earlier, an upset was brewing, but 135th-ranked Jesse Levine of Boca Raton, Fla., wasted a two-set lead and wound up losing to No. 16 Marin Cilic of Croatia 4-6, 2-6, 6-0, 6-3, 6-0.

With No. 3 Rafael Nadal's 6-0, 3-6, 6-3, 6-4 victory over Nicolas Kiefer last night, the top 16 men were 32-0 this week. According to the International Tennis Federation, never in the 41-year Open era have the men seeded 1-16 all advanced to the third round at any Grand Slam tournament.

Serena Williams hasn't had much trouble at all when it comes to the biggest tournaments. She is seeking her fourth championship in the past five Grand Slams, and 12th overall.