Momentum in sports, if you believe it exists, which I do, is a fickle thing. Take today's U.S. Open semifinal match between Roger Federer and Novak Djokovic, for instance.

Federer won the first two sets, and the crowd was loving themselves some of the formerly unstoppable Swiss guy. Should he have beaten the currently unstoppable No. 1 in the world, he'd have been a match away from a major championship that could've served as a power reclamation act.

But today, within the hour, he was up 5-3 in the fifth set. And he was up 40-15 in that game. Two semifinal-match points. Crowd was geeked up. Opponent looked to be on the ropes. The serve was good. The return was ballsily great. Cross-court. No holds barred. If it drops in, it drops in.

It did.

So Djokovic, emboldened and seemingly sick of the crowd willing Federer to win, faced the observers, raised his arms and invited them to recognize the all-or-nothing shot he'd just landed. Are you not entertained? It was then, with a cocky grin, that the Serbian won the crowd's heart in a fashion comparable to what a fictional fighter did in Soviet Russia, on Christmas Day, versus a guy named Drago.

For all intents and purposes, it was over. He fended off another match point and he didn't stop riding the momentum until the fifth set ended with a score of Djokovic 7, Federer 5.

And then, he danced on the court.

On Monday, he'll face the winner of today's Andy Murray/Rafael Nadal match which can't be anywhere as good as the one that just ended. Can it?

Novak Djokovic v Roger Federer - as it happened! [Guardian]