Top seed Kuznetsova was leading Francesca Schiavone 6-2 when the Italian retired with an ankle injury, while sixth seed Dementieva came from 4-0 down to beat Wimbledon runner-up Marion Bartoli of France 6-4 6-2.
But there will not be a third Russian in the semi-finals after fourth seed Dinara Safina self-destructed, blowing four match points before losing 1-6 7-6 6-2 to Greek Eleni Daniilidou.
Daniilidou will play Hungarian teenager Agnes Szavay in the last four after the 18-year-old beat eighth seed Alona Bondarenko of Ukraine 6-1 6-4.
Dementieva looked in trouble when she trailed Bartoli 4-0 but the Russian reeled off 12 of the next 14 games to clinch victory.
Third seed Bartoli struggled with a broken nail but Dementieva was good value for her win as she reached her fourth semi-final of the year.
"It was very difficult to get into the match," Dementieva said.
"I was trying just to find the rhythm and fight for every point and when I found it I just started to play better."
Kuznetsova said there would be no surprises when she and Dementieva meet for the eighth time.
"I played her here last year in the quarters I think and we had a three-setter," she said.
"I always enjoy playing her. I know her game pretty well, as she does mine. I think I have a better record (4-3), but still, it's always good to play your countrywoman and it great to have two Russians in the semi-final."
Safina had two match points on serve at 5-4 in the second set, and then two more in the tiebreak but let it slip.
World number 42 Daniilidou then eased through the third set to win a match that contained 14 breaks of serve.
Szavay followed up her win over second seed Daniela Hantuchova with a confident display against Bondarenko to reach the last four.
"If somebody had told me before the tournament that I would be in the semi-final, I would have been really satisfied," the 18-year-old said.
"I'm very tired but I hope I can prepare well for tomorrow."
