The second-ranked Williams, who was forced to save three match points in a three-set quarterfinal win over Caroline Wozniacki of Denmark late Wednesday, will now head to Melbourne for next week's Australian Open.
"I gave it away more than anything,'' Williams said. "I made a lot of errors and made her look like a champ, really. I just pretty much gave her the match.''
The nine-time Grand Slam champion, however, remained optimistic about her chances at the season's first major, saying she is feeling healthy.
"It was good to have a few matches under my belt getting ready for Melbourne,'' she said. "I feel like I'm going to play a lot better than what I did this week.''
Dementieva will take on third-ranked Dinara Safina for the Sydney title in a reprise of the Beijing Olympic final between the two Russians. The second-seeded Safina beat Ai Sugiyama of Japan 6-4, 7-6 (3).
"I realize how tough it's going to be to play against Dinara tomorrow,'' Dementieva said. "She's in great shape right now. But it's going to be a great experience anyway. This is what we're all here for. We just want to get the best preparation as possible and play our best in the Grand Slam.''
Players have struggled with the heat in Sydney this week, and Thursday afternoon temperatures soared above 104 degrees F (40 degrees C). But loud thunder cracked above during the second set of the Dementieva-Williams semifinal match.
In the men's quarterfinals, top-seeded Novak Djokovic advanced by beating Mario Ancic of Croatia 6-3, 6-4 in a match that was suspended twice by rain. Despite previous crowd problems in Melbourne and Sydney involving Serbian and Croatian fans, there were few problems during the match, although there was extra security and police on hand.
Djokovic will next face Jarkko Nieminen of Finland, who advanced to the semifinals when third-seeded Jo-Wilfried Tsonga was forced to forfeit due to a back injury.
"I decided this afternoon that my back was too much and I cannot play,'' Tsonga said. "I will stay in my bed and I hope I will be better after.''
Richard Gasquet advanced by beating French compatriot Jeremy Chardy 6-2, 7-6 (4) and will face fourth-seeded David Nalbandian of Argentina, who beat former No. 1 Lleyton Hewitt 7-6 (3), 7-5.
Serena back from brink at Sydney International
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Serena Williams, Safina advance to quarterfinals
