With the roof closed at Rod Laver Arena to protect the players and fans from outside temperatures topping 111 degrees, Williams played her best match of the tournament.
"I haven't moved like that for a while, so I was a little shocked," she said.
The second-seeded Williams has a sequence of winning the Australian Open every alternate year since 2003.
She was only a game from a quarterfinal exit on Wednesday when Svetlana Kuznetsova was serving for the match but found her rhythm quickly against Dementieva, who had won their previous three matches.
The 27-year-old American got the only break in the eighth game of the first set and, after falling behind 3-0 in the second, ran off four straight games to get back in front.
After exchanging service breaks again, Williams held to clinch the match in 54 minutes.
After beating Russians in two successive matches, she'll face another one in the final. No. 3 Dinara Safina played No. 7 Vera Zvonareva in the second semifinal.
While it was cooler inside than out at Melbourne Park, Williams already had soaked through her blue dress with sweat by the time the second game was over, not surprising since they had played 16 minutes.
When Williams blasted a clean crosscourt winner while serving at 3-3, she gave Dementieva a long glare. Dementieva smacked a service return winner on the next point and glared right back.
Williams got the only break of the set in the next game when Dementieva hit consecutive backhands wide, then held with the help of an ace.
Dementieva held to start the second set in a game that went to deuce five times and lasted 14 minutes, then broke Williams en route to a 3-0 lead.
That sparked a four-game run for Williams until Dementieva broke to even the second set at 4-4.
Then nerves seemed to get the best of Dementieva. Having problems with her service toss, she double-faulted twice, the second setting up break point. Williams ripped a backhand winner down the line and pumped her fist as she looked toward her mother Oracene and sister Venus.
Williams has had problems with her first serve throughout the tournament, but it came through when she needed it most.
Serving for the match at 5-4, she started with an ace and hit another serve that Dementieva sent long.
At 30-15, Williams followed with another ace. The frustrated Dementieva squealed and bent over in frustration. Another powerful serve on match point set up an easy overhead.
Williams shouted and repeatedly jumped up and down in joy.
Roger Federer, seeking a record-equaling 14th Grand Slam singles title to match Pete Sampras' career record, was playing American Andy Roddick in the night semifinal.
No. 2 Federer, who is 15-2 against the seventh-seeded Roddick, who beat defending champion Novak Djokovic in the quarterfinals.
Top-ranked Rafael Nadal set up an all-Spanish final against Fernando Verdasco when he beat No. 6 Gilles Simon 6-2, 7-5, 7-5 on Wednesday night, when the temperature had dipped to 93 degrees from the high of 109 degrees.
Verdasco ousted 2008 runner-up Jo-Wilfried Tsonga 7-6 (2), 3-6, 6-3, 6-2.
"I think it's incredible for us," Nadal said. "One will be in the finals, so we have to be happy with that."
No Spanish man has won the Australian title.
Wednesday marked the start of what weather forecasters were predicting would be a once-in-a-century heat wave for the city.
Nadal, a Majorca native, was relieved he'd been given a night match, and joked about burning his feet when he went outside to practice earlier in the afternoon.
"Believe me, I never feel the same like today when I was warming up outside," he said. "The conditions were very hot. I couldn't walk."