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
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."
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Images: Pilot Pen Open