Unbeatable in sport is only a temporary truth, it never lasts. Unbeatable is a phase, like Serena Williams on a 25-match winning streak, but it always ends. Unbeatable is a feeling where you think no hardship can't be worn, no opponent is beyond taming, and then life abruptly gangs up on you.

On a noisy Sunday afternoon on Rod Laver Arena, a problem back and a problematic rival was a combination beyond even Williams' grand talent in the fourth round. Ana Ivanovic won 4-6, 6-3, 6-3. Williams, the world No. 1, is now again officially beatable.

To be fair, Williams, once guilty of inelegant quotes when she lost, was repeatedly gracious about Ivanovic. "I thought she played really well", said the American. "She made some good shots. I don't want to blame anything, I feel like Ana deserves all the credit."

To be frank, her locked/stiff/worrisome (take your pick) back was first mentioned not by her, but by journalists. To be clear, of the 25 questions asked to her, 13 were connected to her injury. Never did she use it as an excuse. If she lost in ugly fashion (31 unforced errors), she was graceful in the interview room.

Her back impeded her movement, locked her feet, interrupted her stroke-play. "I obviously wasn't hitting as I normally would hit and wasn't moving the way I normally would move," she said. If we didn't get the point, she added: "I made a tremendous amount of errors... shots I haven't missed since the 1980s."

Certainly it felt that way. In three sets, she had two winners on the forehand and three on the backhand. Often she can do this in a single game. In the last two sets, from both sides, she hit a single winner. This was not unbeatable tennis.

If her body was out of sorts, Ivanovic was beautifully out of character. The American owns a muscular mind, the delightful Serb seems some days to have one made of crystal. She won the French Open in 2008 and has never even been to a Grand Slam semi-final since. Chained by endless defeats, this afternoon she seemed to release herself.

Her serve sang (78 per cent of first serve points won), she rifled forehands (20 winners), she stood inside the court to return second serves (Williams won only 41 per cent of second serve points) and she held tightly onto aggression. Never has she won a set in four meetings with Williams, never has she won more than four games in a set from Williams. Yet today her performance could be distilled to three words. "I actually believed."

Williams is so powerful a figure that her exit entirely alters the women's tournament. No single man's exit can do this on their tour. The remaining ladies challengers - all of whom have dismal records against Williams - will smell hope but must grab opportunity. Williams, or more tellingly Ivanovic, have given them a gift.

To Williams' credit, despite being in pain, despite admitting that before the previous match "I almost pulled out", she played. "I'm a competitor," she stated, in case anyone on this planet was unsure. Then just to reinforce the point, she noted: "Every time I lose I get better."

Champions, even on their worst days, find a sliver of positivity to hold onto. Defeats must have reasons, defeats must be good for them in some way. And so she said: "As I fell at Wimbledon (2013, also in the fourth round), I was able to get myself back up (she won the US Open thereafter). Sometimes it's good when I lose because it takes my head out of the clouds. Not that they are ever there, so to speak. I just work as if I'm ranked 1000 in the world."

And then start another streak. And become unbeatable again. For a while at least.