NEW YORK (Reuters) - Former champion Svetlana Kuznetsova was stunned by Serbian 19th seed Jelena Jankovic 6-7 6-3 6-2 in the fourth round of the U.S. Open on Monday.

Sixth seed Kuznetsova, the 2004 Open champion, had 46 unforced errors and 41 winners in the two-hour, 19-minute match on the Grandstand.

Jankovic, ranked 20th in the world, blasted 37 winners but had only 24 unforced errors in her first meeting with Kuznetsova.

In the quarter-finals, 21-year-old Jankovic will face Russian Elena Dementieva, who defeated France's Aravane Rezai 7-5 6-4 on Monday.

Rezai hung in against fourth-seed Dementieva but was ultimately done in by committing 35 unforced errors during the 79-minute match.

Elsewhere, second seed Justine Henin-Hardenne hammered out a warning to her rivals for the U.S. Open title when she crushed Israeli Shahar Peer 6-1 6-0 to reach the quarter-finals.

The 2003 champion ripped past 21st seed Peer in just 50 minutes to set up a clash with either seventh seed Patty Schnyder of Switzerland or American Lindsay Davenport, the 1998 champion.

"I walked on the court thinking that it could be a dangerous match because she's a young player and she has a lot of motivation," said Henin-Hardenne. "She's fighting a lot. So I knew it could be a tough match.

"That's probably why I won it pretty easily, because I was ready for it."

Peer, who was appearing in the fourth round here for the first time, had no answer to Henin-Hardenne's power and the Belgian cruised to victory.

Henin-Hardenne needed three sets to get past Ai Sugiyama in the previous round but she won 12 of the first 14 points on her way to taking the first set in 23 minutes.

"I had a lot of determination, was very focussed," she said. "I went to the net a lot, did a couple of serve and volleys. I played an aggressive game. So I'm very happy about what happened on the court."

Henin-Hardenne is seeded to reach her fourth successive slam final this year but said she did not want to think that far ahead.

"A grand slam is pretty long," she said. "It becomes harder when you arrive in the quarter-finals because everyone is playing very good tennis and everyone has a chance to go to the end.

"I will just try to go step by step, no matter who's in front of me tomorrow. I will just try to play my game and try to win this one, for sure."