The beginning of the end for Roger Federer?

For most tennis players, two losses in a row is a hiccup, not a slump. But for Roger Federer, a second losing "streak" in four months might represent a chink in the armor.

Federer won his 12th Grand Slam at the U.S. Open last fall, but he's looked rather mortal since. He lost to Novak Djokovic in the semifinals of the Australian Open, and fell in the first round of the Dubai Tennis Championships Monday.

True, the loss to Andy Murray was Federer's first venture onto a court since the loss to Djokovic at the end of January, and he should be forgiven for a bit of rustiness. And Murray is no slouch, especially for a first round opponent.

But I often think about the pressure associated with being the world's top player, especially for such a long period of time -- currently at 213 weeks and counting -- and wonder if Federer ever gets tired of it. It takes a different kind of mental fortitude to stave off all comers than to be gunning for the top.

At 26, Federer is by no means over the hill, and I believe that he has a lot of high-level tennis left in him if he wants it. But if he doesn't want to remain atop the hill -- if Federer decides he's had enough -- there are plenty of others who are waiting hungrily to snare his spot. Djokovic. Murray. Rafael Nadal. Jo-Wilfried Tsonga.

With 12 Grand Slam titles, Federer needs three to surpass Pete Sampras for the all-time mark, and I wouldn't bet against him getting them at Wimbledon this year and next, and probably another hard court title or two between the U.S. and Australian Open in the next few years.

I don't think Monday's loss was the beginning of the end of Federer's reign of dominance -- I think the loss to Djokovic was. Federer may indeed be the greatest of all time, but I'm starting to wonder if his time's almost up.