Jankovic will face Olympic champion Elena Dementieva in the next round, while Dinara Safina will play Vera Zvonareva in the other semifinal match.
"Today she (Jankovic) is the strongest player in the world,'' Dementieva said. "She's playing terrific tennis and has just won back-to-back titles on different surfaces ... It won't be an easy match.''
Dementieva beat Nadia Petrova 6-4, 4-6, 7-6 (6), while Zvonareva defeated Dominika Cibulkova of Slovakia 7-5, 6-4 and Safina ousted Svetlana Kuznetsova 6-4, 7-5,
Jankovic took the top ranking from Serena Williams on Monday after winning back-to-back titles at the China Open and the Porsche Grand Prix in Germany. Jankovic will retain her top ranking next week regardless of this weekend's results because Williams, the runner-up in Moscow last year, pulled out with an injury.
After trading serves twice in the first set, Jankovic broke Pennetta for a 6-5 lead but the Italian broke back to force a tiebreaker. Pennetta was up 3-0, then trailed 5-3, but leveled to 6-6 before returning wide in the next two points.
The 18th-ranked Italian, up 3-2 after breaking Jankovic in the opening game of the second set, saved five break points in the sixth game before Jankovic finally broke her on the 12th deuce in a 30-point game to level 3-3.
"It was the decisive moment of the match,'' Jankovic said. "If I have lost the game, the match could have gone in her direction. It was very important for me to win that game and come back. I was able to stay strong and win that game.''
Petrova went up 4-3 in the all-Russian match with a break in the seventh game of the first set, but Dementieva won three consecutive games to take the set. Dementieva was leading 4-1 in the second set but Petrova came back and won five consecutive games to level the match at one set each.
In the third, Dementieva again took a 4-1 lead but Petrova rallied back to 4-4 and then both players held their serves to force the tiebreaker, which Dementieva won on her third match point.
"It was not an easy match,'' Dementieva said. "There were many missed chances for me today.''
Safin, in a rematch of the 2006 final when Davydenko won his second title in Moscow, made a decisive break in the fifth game of the third set. He closed the match on his second match point when Davydenko netted a forehand return.
It will be only the second semifinal appearance for the former top-ranked Russian, who lost to Roger Federer in the Wimbledon semifinals.
"Much depended on my serve in this match,'' Safin said. "I had few chances against him on the baseline. My only chance was to attack him, risk and hold my serve.''
Safin had 17 aces.
"He (Safin) controlled the match,'' Davydenko said. "I was surprised that Marat has shown his tennis of the best quality.''
Safin will next play Mischa Zverev of Germany, who saved two match points and rallied from 5-1 down in the third set to advance to his first career ATP tour semifinal match with a 6-4, 3-6, 7-5 victory over Viktor Troicki of Serbia.
"At 5-1 down I just relaxed and was almost at home in my thoughts,'' Zverev said. "He (Troicki) was so close to winning that I decided to take advantage of it and attack him. And it worked perfectly this time.''
Fabrice Santoro won the first set 6-3 and was 2-0 up in the second when fourth-seeded Paul-Henri Mathieu retired because of illness.
"The first set was my best performance here so far,'' Santoro said. "I was serving well while he (Mathieu) was obviously out of form and could not return at full strength.''
Santoro, who will turn 36 in December, won his only title this season in Newport, Rhode Island, in July. He will next face Igor Kunitsyn of Russia, who beat Jeremy Chardy of France 6-4, 6-2.
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