"I've been working so hard, and I really, I really wanted it," said Williams, a four-time Wimbledon champion who lost in the first round of the French Open. "I got a little tight in the second set. I couldn't relax. I was like, looking too far in the future and she came back. But I'm glad I was able to get through."
The previous Wimbledon record of 23 aces was also set by Williams, in a three-set victory over Zheng Jie in the third.
Williams controlled the play against Azarenka in the first set with her service game, winning 20 of the 24 points she started. She then went up an early break in the second set, but Azarenka responded to make it 3-3.
They held the rest of the way, and Williams picked up her final three aces in the tiebreaker.
Williams will face Agnieszka Radwanska in Saturday's final and attempt to tie her sister Venus by winning her fifth Wimbledon singles title. The third-seeded Pole beat Angelique Kerber 6-3, 6-4 in the other semifinal.
Radwanska can take over the No. 1 ranking if she beats Williams on Saturday. If not, Azarenka, who won the Australian Open, will take over the top spot from Maria Sharapova.
"I'm just going to try to mix it up," said Radwanska, who cut short her post-match news conference because of a coughing fit. "Of course, every match is different, so I'll see after tomorrow how it's going to be."
In the men's semifinals Friday, defending champion Novak Djokovic will face six-time winner Roger Federer, while Andy Murray will take on Jo-Wilfried Tsonga.
For Radwanska, it was a steady hand or steady backhands and forehands actually that put her into her first Wimbledon final.
She won five straight games to take the first set, and then held on after taking an early lead in the second.
"This is a dream from when I was kid," the 23-year-old Radwanska said. "I'm playing tennis almost 18 years, and of course everybody's dream is to play the final of a Grand Slam."
Radwanska is the first Polish woman to reach a major final since 1939.
P Men's semifinals, 6 a.m.
TV • ESPN