Mats' Musings: Don't count out del Potro

Editor's note: Former world No. 1 and seven-time Grand Slam champion Mats Wilander is writing exclusively for Yahoo! Sports throughout the U.S. Open. The 1988 U.S. Open champ breaks down who's hot and who's not, offering his thoughts courtesy of Lacoste.

The departure of Andy Roddick from the U.S. Open on Saturday night signaled to most fans the end of any chance that the men's singles winner will come from outside the Big Four.

Before he was dumped by John Isner, Roddick seemed to be the only player with any realistic chance of breaking the monopoly that Roger Federer, Rafael Nadal, Andy Murray and Novak Djokovic hold over the sport.

However, there is still one outsider worth keeping an eye on – the fast-improving Juan Martin del Potro.

The Argentine has made great strides in the past year and has a big-time game that is capable of causing anyone problems.

Del Potro has huge power off the ground and has elevated his service to elite level. Now he needs to find the right level of consistency to get through four more matches.

Del Potro still looks skinny, but he has put on plenty of muscle over the past year and the difference in his game is obvious. He is able to take the ball early, striking it with remarkable force.

There is a strong chance del Potro could meet Andy Murray in the quarterfinals in what would be a fascinating meeting of two of the game's brightest stars.

Here's another glance at the favorites for the men's title, with Roger Federer still looking good for a sixth straight New York crown.

MATS WILANDER'S YAHOO! SPORTS POWER RANKINGS

1. Roger Federer: Finding it easy while conserving energy.

2. Novak Djokovic: Looking sharp and helped by Roddick's exit.

3. Andy Murray: Class act who can go all the way.

4. Rafael Nadal: Looking good but bothered by some niggling injuries.

5. Juan Martin del Potro: Tall South American can't be ruled out.