Venus, playing at night, looked strong against Daniela Hantuchova in winning her 10th consecutive Grand Slam match, Serena, in the daytime, struggled against a stiff wind and the spins and slices of veteran Italian Francesca Schiavone.
After three magnificent efforts, Venus ought to be a heavy favorite when the sisters meet tomorrow for the 14th time, dating to the 1998 Australian Open.
But this is Williams against Williams, which always seems to fall into the category of "go figure," though there may be, for the first time, a real revenge factor in this reluctant rivalry.
The last time they played, in the quarterfinals of the Nasdaq-100 on Key Biscayne in March, Venus broke a six-match losing streak to her younger sister, winning 6-1, 7-6 (8).
But the younger Williams has some asserting to do after a season of injury, and it has to begin in the next round.
"I'm playing OK. I'm definitely better than my first match; definitely better than I was even a week ago and two weeks ago," she said after her 6-3, 6-4 win over Schiavone, who defeated her in the Italian Open in May.
But will it be enough to overcome the far more consistent play of her sister? Serena's forehand continues to be erratic and there were another couple of badly blown short balls. In contrast, Venus was deadly on any ball she took near the service line in her 6-3, 6-3 triumph over Hantuchova.
They long ago put away any problem with these matches. Now, they seem more playful than anything else, with Serena suggesting she'll poison her sister's food and Venus responding, "She doesn't know I have a food taster."
After two days of upsets, with defending women's champion Svetlana Kuznetsova and America's newest men's icon, Andy Roddick, going out in the first round, the tournament has settled into a comfort zone.
There was only one tight spot in Venus' victory. She gave a break back to tie the opening set at 3 when the much-improved Hantuchova hit a defensive service return and induced Williams into a wild error on break point.
But Venus broke back immediately as Hantuchova committed three unforced errors. On set point, Williams tracked down a forehand, measured the shot and slugged a crosscourt pass.
Off Serena's first two matches, Schiavone had expectations of an upset, but she played a very disappointing match. She double-faulted six times without an ace and converted only one of three break-point opportunities.
This was not great offense from Serena, who looked more intent on just getting into a rhythm with her Italian opponent.
"It's obviously disappointing to have to play my sister in the next round," she said.
"If I say yes, then that means I'm saying she's vengeful," Venus remarked when asked if her sister is capable of seeking revenge. "I don't know. I really don't know. I can't say."
Serena leads the series, 7-6.
In men's matches, No. 3 Lleyton Hewitt won in straight sets, unseeded Paradorn Srichaphan upset No. 6 Nikolay Davydenko, No. 11 David Nalbandian won a four-setter against Peter Wessels, unseeded Novak Djokovic beat No. 22 Mario Ancic, and No. 25 Taylor Dent became the only American to reach the third round in the top half of the draw, beating Nicolas Almagro, 6-3, 6-2, 4-6, 7-5.
Dent and Hewitt, the 2001 men's champion, will play their third match this year. Dent beat the Australian in his hometown of Adelaide, and Hewitt won at Wimbledon.
"I know what I'm going to get," Hewitt said. "It's just a matter of executing, going out there with a game plan and executing what I need to do out there. It's never easy playing against Taylor. He's going to give me a target and I've got to take my chances when I get them."
The South Florida Sun-Sentinel is a Tribune Publishing newspaper. The Associated Press contributed to this article.
Men's singles (top 20 seeds)
L. Hewitt (3) def. J. Acasuso, 7-6 (6), 7-6 (3), 6-2. P. Srichaphan def. N. Davydenko (6), 6-4, 7-5, 6-3. D. Nalbandian (11) def. P. Wessels, 7-6 (4), 1-6, 6-2, 7-6 (4). Dominik Hrbaty (15) def. Michael Lammer, 6-1, 6-1, 6-3. N. Kiefer def. R. Stepanek (16), 4-6, 4-6, 6-4, 6-3, 6-3. David Ferrer (17) def. Karol Kucera, 6-3, 6-0, 6-2.
Women's singles (top 20 seeds)
Maria Sharapova (1) def. Julia Schruff, 6-2, 6-4. Kim Clijsters (4) def. Ai Sugiyama (30), 6-1, 6-4. S. Williams (8) def. F. Schiavone (25), 6-3, 6-4. Nadia Petrova (9) def. Laura Granville, 6-2, 7-5. V. Williams (10) def. D. Hantuchova (20), 6-3, 6-3.
Today's top matches
James Blake vs. Rafael Nadal (2) Andre Agassi (7) vs. Tomas Berdych (32), Guillermo Coria (8) vs. Robin Soderling Richard Gasquet (13) vs. Ivan Ljubicic (18) Nathalie Dechy (15) vs. Tatiana Golovin (23) Sebastien Grosjean vs. Tommy Robredo (19)
Anabel Medina Garrigues (32) vs. Lindsay Davenport (2) Anna-Lena Groenfeld (31) vs. Amelie Mauresmo (3) Elena Dementieva (6) vs. Anna Chakvetadze (29) Justine Henin-Hardenne (7) vs. Cho Yoon-jeong, Shinobu Asagoe (24) vs. Patty Schnyder (11) Jelena Jankovic (17) vs. Mary Pierce (12)