The second semi-final will be between Kuznetsova and unseeded Tatiana Golovin of France, after she beat Ukrainian Kateryna Bondarenko 6-3 6-4 with minimal fuss.
Henin, playing only her second match since winning the U.S. Open on Sept. 8, looked at her untouchable best as she moved 5-0 ahead against Dementieva.
The winner of seven grand slam titles was uncharacteristically wasteful, however, and she twice missed chances to serve for the set.
Dementieva was going for her shots, hitting much more powerfully than at the start, but after getting both breaks back the doubts set in again and she double-faulted on set point.
Henin quickly went a break ahead in the second before again missing the chance to serve out at 5-3. This time she broke immediately to seal victory in 92 minutes.
"I lost a little bit of intensity at the end of each set," said Henin, who will sit out next week's Kremlin Cup in Moscow to let a shoulder problem heal completely. "She's a real fighter. If you give her a bit she'll take it all."
Jankovic only had to play one serious set in her quarter-final.
Petrova had edged it 7-5 on the tie-break before suffering the recurrence of an injury to her left hip, eventually bowing out when 5-1 down in the second.
"I was really surprised she didn't play on, but I know how it feels to play with pain so I wish her a speedy recovery," Jankovic said at a news conference.
"Now I'll go after Henin. I've got nothing to lose."
Like Henin, Williams was playing in her first tournament since the U.S. Open. Defeat will not help her chances of making the season-ending tournament in Madrid.