Jankovic has a 3-2 career record over Williams

Williams began typically slowly and the Serbian broke in the opening game, which was enough to take the first set.

The American 26th seed played more aggressively and cut out the unforced errors in the second set as the match developed into an absorbing struggle.

But Jankovic, 22, struck at 1-1 in the decider and then broke again to win.

French Open: Jelena Jankovic v Venus Williams

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"I never had it in my mind that I would lose," said Jankovic, who came into the tournament as the form player after triumphing in Rome and posted her third straight win over Williams.

"I was a little bit tired in the second set, but I just went for it in the third and it paid off.

"It was an amazing third-round match and it was a shame that Venus had to go out so early. It's always good to beat one of the Williams sisters."

Williams has failed to get beyond the quarter-finals at Roland Garros since reaching the final in 2002.

I don't feel like she came out there and really beat me Venus Williams

The 25-year-old has not reached a major final since winning the last of her five Grand Slam titles at Wimbledon in 2005.

"I got a little bit tired at the end," said the American.

"The third set went a little bit faster than what I expected. But am I discouraged? No, not at all.

"I don't feel like she came out there and really beat me. I just feel like at times she was a little more patient than I was."

And Williams insisted she would not be tempted to play any warm-up tournaments ahead of Wimbledon.

"I really feel good about my game. I would like it to click at Wimbledon," said Williams.

"I feel good to have conquered all the things that I've conquered.

"I just went long, long, long but it's only a matter of time before it goes in, in, in.

"I definitely feel like I could have done things to be in the winner's circle. I don't feel like I went out there and didn't have any courage.