It Is amazing how often a player's words come back to haunt them.

On the eve of the Australian Open, Serena Williams had said: "I just have to stay out of my way and I'll be fine." Yesterday, the world No.1 fell victim to a resurgent Ana Ivanovic in the fourth round, beaten 4-6, 6-3, 6-3, exiting before the semi-finals for the third straight year and blowing the draw wide open.

It was a brilliant performance from Ivanovic but inevitably questions were raised as to the fitness of Williams. The answer came quickly. "She has had back problems since the day before the [third-round] match against [Daniela] Hantuchova," Patrick Mouratoglou, Williams' coach, revealed, appearing to almost enjoy the spotlight, even as Williams began her press conference nearby. "She blocked her back. I don't know how she won against Hantuchova because it was 10 times worse [then] than today.

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"Today it was a combination of the fact that she couldn't move the way she usually moves, so she was in a bad position to hit the ball most of the time and couldn't be really aggressive, and the fact that Ana played a great match."

Williams was disappointed that news of her back injury crept out. "Oh, so he's the one who's snitching," she said, to much laughter, not much of it hers. The American has never given much away when she's been injured, fearful of taking the gloss off her opponent and looking like a sour loser. It is an attitude that can be frustrating but with the cat out of the bag, the 32-year-old admitted that she had been struggling for a few days and that she had been close to pulling out before playing Hantuchova.

Given all that, it shows how far ahead of the rest Williams is that apart from a little sluggishness, there were few visible signs she was having trouble, although the unforced error count, in retrospect, was a giveaway.

Still, when she recovered from an early break down to take the first set against Ivanovic, it looked like being business as usual. It was from then on that Ivanovic, who won the title in Auckland at the start of the year, began to play the kind of tennis that took her to the No.1 ranking and the French Open title in 2008.

The Serb struck her forehand with impressive authority and even her serve, which has been her Achilles heel for the past few years, held firm. It was a fine performance and, as Williams' final return sailed long, she let out a long yell of joy.

"I made a tremendous number of errors, shots I missed I normally don't miss," Williams said, "shots I haven't missed since the '80s. Maybe I wasn't the best physically, but that had nothing to do with it. I think Ana just played a really good match. She did what it takes to win."

For Ivanovic, not only is she now two wins away from another grand slam final, it could prove to be a career-changing victory. The 14th seed has spent the past few years hovering between No.10 and 20 in the rankings but now could easily find herself back in the top 10, having lost her way almost as soon as she hit top spot.

"I'm still a little bit shy, so for me it was overwhelming with all the success and attention I got," she said. "I kind of wanted to get away from it a little bit. And then when I was away, I didn't want that. I wanted to still work hard. But then I was not in the best place in my mind. But you keep fighting because this is what I love to do and this is what I'm best at. I still am so young and I deserve a better chance and better shot at it."

In the quarter-finals, Ivanovic will play Canada's Eugenie Bouchard, who beat her at Wimbledon last year, while the other last-eight match in the top half of the draw will be between China's Li Na and Italy's Flavia Pennetta.

By contrast, the men's event is motoring along with very few shocks and, as he usually does in Melbourne, Novak Djokovic is looking close to unbeatable. So confident is the Serb about his game and ability that he managed to watch the last set and a half of Ivanovic's win, before then taking apart Italy's 15th seed, Fabio Fognini, 6-3, 6-0, 6-2. It was as ruthless as it sounds - perhaps even more so - and took his winning streak to 28 to boost his confidence even more.

"I've been elevating my game as the tournament is going on," he said. "Every match is better. There is this confidence that I carry on from many wins that I had in the last two months of the 2013 season and I started off this season in a good style."

Standing between him and the semi-finals is Stanislas Wawrinka, who beat the Spaniard Tommy Robredo 6-3, 7-6, 7-6 with some stunning hitting. Having pushed Djokovic to 12-10 in the final set of an epic encounter last year, perhaps he may yet have a say in the destination of the title.

Seventh-seeded Czech Tomas Berdych has not dropped his serve, let alone a set, in four matches, having dispatched South African Kevin Anderson 6-2, 6-2, 6-3 yesterday. Third seed David Ferrer dropped his second set of the event but came through 6-7, 7-5, 6-2, 6-1 against the tricky German Florian Mayer.