(CNN) -- World number one Jelena Jankovic will face defending champion Elena Dementieva in the Kremlin Cup semifinals after both players had to fight hard to overcome their unseeded opponents on Friday.
Top seed Jankovc won five successive games in the second set to reach the Kremlin Cup semifinals.
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 both the second and third sets but allowed her compatriot to come back each time.
But the Olympic champion, who also beat Petrova in three sets in this year's Wimbledon quarters, was not to be denied as she held her nerve to clinch the tiebreak 8-6 after nearly three hours. "She changed tactics in the second set and played more aggressively," said Dementieva.
Top seed Jankovic, making her Moscow debut and the only non-Russian left in the tournament, also had to use all her big-match 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 to secure victory.
"It was a tough match for me today and I didn't play it the way I wanted to," said the 23-year-old Jankovic, who had received on-court treatment for a sore back in Thursday's play. "I felt a bit tired and was struggling myself all the way."
Seventh-seeded Russian Vera Zvonareva is also through after beating Slovak teenager Dominika Cibulkova, who upset world number five Ana Ivanovic in the previous round, 7-5 6-4.
The in-form Zvonareva will now face compatriot and second seed Dinara Safina, who proved too strong for another Russian, Svetlana Kuznetsova, winning 6-4 7-5.