Defending champion Serena Williams joined her sister, Venus, in the third round of the US Open with a 6-1, 6-1 victory over Melinda Czink of Hungary.

Williams then offered a new, tongue-in-cheek explanation for why she plays so much better at grand-slam tournaments than other events. "Maybe it pays more," she said with a smile. "I guess I'm trying to pay off my mortgage."

Williams put together a near perfect match to beat 51st-ranked Czink in less than an hour as she seeks her fourth US Open championship and 12th grand-slam singles title overall. She's won three of the past four major tournaments, but she has failed to win a single non-major tournament in that time.

The American is trying to become the first woman to win consecutive titles at Flushing Meadows since Venus did it in 2000-01. The siblings could play each other in the semi-finals.

Venus wore heavy bandages above and below her left knee while winning earlier in the day, eliminating Bethanie Mattek-Sands of the United States 6-4, 6-2.

Serena played Czink at a hard-court tournament in California in July, and things were much more competitive until Williams eventually prevailed 6-3, 7-6. "I definitely made adjustments [for this match]," Williams said. "I knew her game better today."

Czink had trouble handling Williams's power as the American finished with an impressive ratio of 25 winners to only nine unforced errors. About the only glitch for Williams came midway through the first set, when she thought she had pounded an ace. Instead, the serve was waved off by a line judge, who called a foot-fault. Williams turned and glared, setting her hands on her hips. She just stood there with a straight face for a few seconds, staring down the official. When play resumed, Williams pushed a forehand into the net, then again sent a serious look in his direction.

On the following point, Williams hit an ace at 115 mph and looked over once more. "I was kind of frustrated because I hit a great serve, the best serve I hit in months," Williams said.