Chris McGrath: Wayward Venus spirals out of control against world No 82

All these anonymous women are suddenly in danger of making a name for themselves. As though goaded by constant charges of mediocrity and stagnation, the incognitos yesterday broke cover so boldly that three of tomorrow's semi-finalists have between them mustered one previous appearance in the last four of a Grand Slam. And those who conclude that the fourth, Serena Williams, can now help herself to her 13th such tournament, from a silver platter, do no justice to their collective courage and craft. Serena need only ask her own sister.

For if it required imagination even to entertain the possibility that Tsvetana Pironkova might beat Venus Williams, then predicting that she would do so 6-2, 6-3, might have warranted incarceration. Ranked 82 in the world, the 22-year-old Bulgarian had managed to win only one match here in four previous visits. Her opponent, of course, had made eight of the last 10 finals, winning five, and was seeded No 2 for a third consecutive shoot-out with Serena.

And while Venus was unmistakably short of her best, her performance strewn with unforced errors, Pironkova never looked like letting her off the hook. Even if the cornered tiger proved toothless for once, it still took nerve to approach for the kill. Broken in the third game of the second set, Pironkova responded in the very next game by salvaging a drop shot and when the lob landed inside the line the crowd went wild. That set up the chance to retrieve the break, and consecutive double-faults from Venus in her next service game left her 2-4 down. She saved two match points at 3-5, but Pironkova served out to complete the most stunning result of her career in 85 minutes.

Venus produced the last of 29 unforced errors at match point – dabbing a simple forehand volley into the net – and had managed just four baseline winners. "I just let it spiral and didn't get any balls in," she said. "I think I missed all shots today: forehand, backhand, volley. If there was a shot to miss, I think I missed it. She played really well, but maybe not as tough as my fourth round or my third or even second. I wasn't overpowered, hit off the court or anything... I just let myself exit."

Some day, inevitably, this kind of performance will disclose a fatal erosion of her competitive longevity. At 30, however, she fiercely resists any anticipation of that moment. "Well, why wouldn't I want to pursue this?" she asked. "I'm pretty good at it most days. Today I didn't seem to be the best tennis player, but for the most part I rock and roll this game."

As it happens, Pironkova had also caught her out in the very first round of the 2006 Australian Open, but the Bulgarian had prepared for this tournament by failing to qualify at Eastbourne, beaten by the British teenager, Heather Watson. In fairness, she is entitled to keep developing on grass, having sampled the surface for the first time only in qualifying at Roehampton five years ago. "Back then I thought: 'Wow, this is impossible! How can I play on this?' But with every match I play on grass I feel better and better. Coming here, I just wanted maybe to win one or two rounds. But semi-final looked to me very far. This is truly like a dream to me."

She now meets Vera Zvonareva, who herself saw off a rival of much stronger Grand Slam pedigree in Kim Clijsters, 3-6, 6-4, 6-2. Clijsters conceivably remains a little rusty on grass, after ending a premature retirement last year, but Zvonareva showed resilience, elegance and imagination to shade an entertaining match. The Russian did make the last four at Melbourne last year and, ranked as high as No 5 last year, is climbing back up the rankings. But she lost her only previous meeting with Pironkova, and will hardly be complacent in her relative familiarity with these altitudes.

Clijsters observed that the game had changed during her absence. "I really feel there are a lot of girls out there who can push the top players really hard," she said. "Maybe not on a consistent basis. But that's what we're seeing here. It's good to see new names doing well at the Grand Slams. I haven't seen them all play, but obviously they can hit the ball hard. You don't get to a quarter- or semi-final by luck."

Step forward the unseeded Petra Kvitova, who confirmed herself the revelation of the tournament in edging past Kaia Kanepi, 4-6, 7-6, 8-6. The Czech stared down the barrel several times, saving five match points, but now has the chance to become the first left-hander to win here since her compatriot, Martina Navratilova, in 1990. Eight days ago she had never won a match on grass, but she has certainly earned her passage, already seeing off Caroline Wozniacki and Victoria Azarenka. At 20, however, she will need all the insolence of youth against Serena, who is still to drop a set after seeing off Na Li 7-5, 6-3.

The champion professed herself unsurprised by her next opponent, having been impressed by her through the tournament. Understandably, however, she seems pretty happy with her own game. "I've never served this well," she admitted. "I always serve well here, but this is the first time I've served this well so consistently. But it's not mine to lose. It's mine to win, if I can get it. There's three others vying to win it. They have as good a chance as I do."

The bookmakers won't buy that and nor, perhaps, does Serena herself. But her sister just might.