At the mixed team tennis championship, Williams gained revenge against Belgium's Kim Clijsters with a 7-5, 6-3 victory before teaming up with James Blake for a 7-6 (8-6), 3-6, 10-5 triumph over Clijsters and her countryman Xavier Malisse.
The tournament uses a "super" tiebreaker, in which the first team to 10 points that wins by at least two points wins the match.
There were eight breaks in the first set of the women's singles, but Williams broke in the 12th game and held her own to go up, 7-5. She jumped out to a 3-0 lead in the second set and cruised to victory over Clijsters, who had won their last meeting in the final of the season-ending WTA Championships at Los Angeles in November.
"I definitely stepped it up a bit today," said Williams, the reigning French Open, Wimbledon and U.S. Open champion. "It doesn't matter who I play or who I beat. In the end, you're going to have to play everyone, whether I'm playing Venus (Williams) or Kim in the final or in the quarterfinals. It doesn't matter who you're going to play."
Malisse squared the Group A encounter at a match apiece by beating Blake, 7-6 (8-6), 6-2. The Belgian needed three set points to clinch the first set, but won the last four games to wrap up the second set.
"I think that this is the best match I've played since I've gotten here," Malisse said. "I didn't make any unnecessary errors, just a couple. I served very well. I think (it was a) really good match. I got a little lucky in the tiebreak. In the second set, I played better on the break points I had. I played a little more safe."
The United States now faces Australia in the final, and Williams looked ahead to her singles match against Alicia Molik.
"She's playing really, really good here," Williams said. "She's really sneaking up on everybody and doing really well and maybe even underestimated a bit, but I never underestimate my opponents and I really expect to have a good match with her tomorrow."
Blake will take on men's world No. 1 Lleyton Hewitt, who beat the American twice in tight matches at the U.S. Open the last two years.
"Unfortunately, I haven't quite gotten a win out of either of those," Blake said. "First time, unfortunately I got sick on the court. Second time, he just got one break on a couple of great shots. He's good at doing that. That's why he's No. 1 in the world. But I feel like I have a pretty good game to match up with his and I think I've got as good a chance as a lot of guys on tour to beat him."
Both the United States and Australia will be seeking their second Hopman Cup title.
In another Group A tie, Spain beat Uzbekistan (8), 2-1. Iroda Tulyaganova of Uzbekistan topped Virginia Ruano Pascual, 7-6 (8-6), 6-0, but Spain's Tommy Robredo routed Oleg Ogorodov, 6-0, 6-4, before pairing with Ruano Pascual for a 6-2, 6-4 triumph over Tulyaganova and Ogorodov.