By Steve Ginsburg

NEW YORK (Reuters) - Jelena Jankovic exploited Elena Dementieva's weak serve to upset the fourth seed 6-2 6-1 on Tuesday and advance to the semi-finals of the U.S. Open.

Russian Dementieva, a 2004 Open finalist, produced five double faults as she failed to hold any of her seven service games against her 19th-seeded Serbian opponent.

"She was playing great from the beginning to the end," said the 24-year-old Dementieva. "She didn't give me any chances. I feel like she was better today. For me, it was very hard.

"I was fighting for every point, but she was just better and better. It's just unbelievable, the way she was playing today."

Jankovic next plays either French Open champion Justine Henin-Hardenne or former U.S. Open champion Lindsay Davenport.

The 62-minute triumph over Dementieva in Arthur Ashe Stadium marked her third straight victory over a top 10 opponent.

"It's an amazing feeling," said Jankovic, who beat 10th-ranked Nicole Vaidisova in the third round and number seven Svetlana Kuznetsova in the fourth.

"I don't know how to explain it.

"You can only know this feeling when you really are there and experience it. It's a great feeling to be in a semi-final, especially in Arthur Ashe, in a big stadium. Amazing."

REMARKABLE TURNAROUND

The last four berth caps a remarkable turnaround for 21-year-old Jankovic, who endured a miserable stretch this summer when she lost 10 straight matches.

The Serbian, who lost a three-setter to Dementieva in the finals of the Los Angeles Open last month, said that in the past she has had trouble handling the Russian's 80 mph serves.

"I can return easier Serena's (Williams) serve than Dementieva's serve," said Jankovic, who beat Williams in straight sets in the Los Angeles semi-finals.

"When I played in LA, Serena would hit a 125 mile-an-hour serve, and I would hit it even harder back, a winner.

"But then I played Dementieva, who hits an 80 mile-an-hour serve, and I can't even make it over the net. I don't know what it was.

"But now I've got used to it somehow. I found my rhythm. I think when I was returning well today, I put a little bit more pressure on her."

Dementieva had more winners than Jankovic, 16-6, but also suffered 39 unforced errors compared to just 16 for the Serb, who had never reached a grand slam quarter-final before.

"It's disappointing," said Dementieva. "I'm getting older, and I haven't won a grand slam, so that's really what I'm thinking about all the time. I feel like I was in a good shape here. It's sad."