When Serena failed to convert 11 of the 12 break points!

The World No 1 fights back against France's Kristina Mladenovic after being dragged into a two hour 11 minute tussle.

Serena Williams is not one for wasting time or energy.

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After all, in her first two matches at this year's French Open, she had dropped a total of five games and spent a brisk one hour 48 minutes on court.

With her next opponent, France's Kristina Mladenovic, having never progressed beyond the third round at Roland Garros, Williams was hoping for another easy workout on Saturday.

There was one problem though -- the real Serena failed to show up for the match.

The imposter who did turn up got dragged into a two hour 11 minute tussle in which she failed to convert 11 of the 12 break points that fell her way.

"I feel like I made a tremendous amount of errors. I was playing really defensive. It's not me," the American world number one said after escaping with a 6-4 7-6(10) victory.

"Those break points, most of them I should have won and I didn't."

The only time Williams managed to crack Mladenovic's serve was at the end of the first set.

As nine further chances came and went in the second set, fans inside and outside Court Philippe Chatrier could hear her mounting frustration as she yelled, squealed and roared at her missed opportunities.

"She played well," the 34-year-old Williams said tipping her hat to Mladenovic.

"I feel like she kind of forced me to go for it and, unfortunately, I wasn't hitting great today. She kind of forced the issue."

By the time a thunderous downpour forced the players off court at 6-6 in the second set, the outcome was on a knife edge.

When the players resumed their battle following the 2-1/2 hour break, Mladenovic fed off the hollering crowd's energy to romp into a 5-2 lead in the tiebreak.

But in typical Williams fashion, the American burst the boisterous celebrations by winning the next four points.

What followed was anything but typical Williams.

She produced some howlers as four match points went begging.

At 6-5 Williams drew a chorus of 15,000 gasps as she smashed the ball metres over the baseline and was again way off target at 8-7, misfiring a forehand well beyond the court markings.

The American then drew on the wealth of experience that has earned her 21 grand slam titles to save a set point before finally sealing victory when Mladenovic whipped a forehand wide.

"Up until that point I had not been playing my game. I was playing really defensive. It's not me," said Williams, who will next face Ukrainian 18th seed Elina Svitolina.

"So I just wanted to be Serena out there."