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The American, seeded 10th, looked set for a comfortable win when she took the opening set with plenty to spare.

But world number 94 Pironkova, playing her first Grand Slam match, came storming back to level.

And after a succession of service breaks in the decider it was the teenager who came through 2-6 6-0 9-7.

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"It happens to the best of us," said Williams.

It's the beginning of the year and I feel like I still have a lot of success ahead of me Venus Williams

"I had so many unforced errors, I struggled to keep the ball in. I just couldn't get it right - she benefited from my largesse.

"If I had just one-third less errors, this match is a different story."

Williams served for the match at 6-5 in the third but made an astonishing 65 unforced errors in total as she succumbed to the highly-rated Pironkova.

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"I just seemed to get to a point there, I just couldn't pull my game together," said Williams.

"I just don't know what happened, I don't know, so I'm really disappointed but I feel like, in general, I was in good form. I just somehow fell off my form.

"But it's the beginning of the year and I feel like I still have a lot of success ahead of me. It's just my first event."

Pironkova shocked Venus in her first-ever grand slam match

Pironkova admitted she was not overawed by playing Williams and said: "I have known about Venus for a long time but when I go on court I am a professional so I should not think she was kind of my idol before," she said.

"I was a little nervous at the start, then in the break between the sets I told myself to relax and play my game.

"After one day I have another match so I don't think I should celebrate."

Ninth seed Elena Dementieva was the first big name to fall as she went down 7-5 6-2 to Germany's Julia Schruff, who had never previously won a match at the year's first Grand Slam.

Schruff was in control throughout and held her nerve despite letting a 5-1 lead slip in the first set.