The tournament's two-time defending champion is 12-0 in her three trips to New Haven.
Dementieva has been to these semifinals four times and lost all four.
The Russian appeared to be cruising early, breaking Wozniacki in her first two service games of the match and again in the final game of the first set.
But the Danish star responded with a service break of her own in the opening game of the second set, and won the set when neither player could hold serve during the final four games.
Dementieva got an early lead in the final set and was serving for the match at 5-4.
“I just told myself, “Keep fighting,” Wozniacki said.
“She still has to win four more points to win the match so you're still in there.” Wozniacki took that game, then went up 6-5. But she couldn't finish off Dementieva, who fought off three match points, and went up 3-0 in the tiebreaker before Wozniacki rallied for the win.
After winning last week in Montreal and making another run here, Wozniacki said she's not feeling the pressure of being the top seed when the Open gets under way next week.
“Pressure is when you're put in a spot and you don't really feel like you belong there,” she said. “When you really believe that you belong there, that you can do the things you have to do, there's no pressure. You just go out there and play.” She will face Russian Nadia Petrova in Saturday's final.
The No. 19 player in the world, who was a last-minute wild-card entry to the tournament, beat Maria Kirilenko 2-6, 6-2, 6-2.
Kirilenko broke Petrova twice in the first set, but said she felt a pain in her back after lunging for a ball. She took a medical timeout in the second set and had a hard time moving.
“I made a very fast movement and then I start to feel it even more,” she said. “Then I couldn't even bend at all.
Then it went a little bit better when the (trainer) came out. But still I decided to continue because no reason, you know, just even if you have a pain, you have to try.” Kirilenko said she would have her back looked at before deciding her status for the US Open.
Petrova found her serve in the second set and cruised from there.
Petrova received the final entry to the tournament after Ana Ivanovic withdrew last week with an ankle injury. She called the opportunity to play a “great gift” and has turned it into her first finals appearance since 2008.
In the men's draw, Sergiy Stakhovsky moved into the finals with a 6-3, 6-4 win over Thiemo de Bakker of the Netherlands.
The ninth-seeded Ukrainian took advantage of the only break point in the first set and broke de Bakker again in the seventh game of the second set.
Stakhovsky has played in four semifinals in his career and has won all of them. He is 3-0 in finals, including in Hertogenbosch this year, and is hoping that streak continues Saturday.
“I can't really say what it takes,” he said. “It just takes, I don't know what, but I'm trying to get that.” Stakhovsky will play Denis Istomin of Uzbekistan in the final. He beat Serbia's Viktor Troicki 7-6 (10), 3-6, 6-2.
Istomin said he was having trouble seeing the ball coming out of the sun and into the shadows of the stadium court through the first two sets, but played better in the third.
He hit a drop shot in the sixth game to break Troicki, and broke him again in the final game to reach his first ATP final. After the match, he credited part of his success to his coach, who also happens to be his mother.
“For now I don't know if I can practice with other coaches because I feel very good with Mom, really understanding together,” he said. “It's very good.”