The second semi will be an all-Russian affair between second seed Dinara Safina and number seven Vera Zvonareva.
Safina edged fifth-seeded compatriot Svetlana Kuznetsova 6-4 7-5 while Zvonareva beat teenage Slovak Dominika Cibulkova, who upset world number five Ana Ivanovic in the previous round, 7-5 6-4.
Dementieva was the first player to reach the last four, eliminating fellow Russian Nadia Petrova 6-4 4-6 7-6.
The third seed won the opening set and led 4-1 in the second and third before allowing her compatriot to rally each time. However, the Olympic champion held her nerve to clinch the tiebreak 8-6 after a match lasting nearly three hours.
Jankovic, making her Moscow debut, also had to use all her experience to beat Italian Flavia Pennetta 7-6 6-3.
Pennetta led 3-1 in the second set before the Serb, who replaced Serena Williams at the top of the WTA rankings on Monday, reeled off five successive games.
Tired Jankovic
"I'm feeling a bit tired as I've been playing a lot recently but I've got two more matches left here so I'll try my best," said Jankovic, who won titles in Stuttgart and Beijing in the past two weeks.
The in-form Safina came back from 5-2 down in the second set to seize her fourth consecutive victory over former U.S. Open champion Kuznetsova.
Men's fourth seed Paul-Henri Mathieu, who won here in 2002, retired from his all-French quarter-final against Fabrice Santoro because of illness while trailing 6-3 2-0.
World number 98 Mischa Zverev reached his first ATP semi with a thrilling 6-4 3-6 7-5 win over Serb Viktor Troicki.
The Moscow-born Zverev, who moved to Germany at the age of four, saved two match points as he battled back from 5-1 down in the decider.
"I've lost matches after throwing away a 5-1 lead but I've never won from 5-1 down in the third set so this is a first for me," he told reporters.
Zverev next meets holder Nikolay Davydenko or former world number one Marat Safin, who were playing later on Friday.
