NEW YORK - Andy Roddick has departed, and apropos of nothing but pertinent to everything, Matt Leinart could be arriving, although the belief is he'll end up in another town.

Two young athletes, two different sports, two levels of frustration.

For Roddick, 28, an ancient age in tennis, the best days may be in the past.

For Leinart, 27, and that's getting along in pro football, the best days may never be.

And so again we study the vagaries of sport, the difficulty of predicting who will meet expectations. And who won't.

Roddick was knocked out in the second round of the U.S. Open tennis championships the other day, losing to a Serb with dark glasses and a dark beard, Janko Tipsarevic, which might be important or might not be.

Not the glasses and beard but rather the fact the Serb, Tipsarevic, is 44th in the world rankings, and shouldn't be beating the No. 9 guy, Roddick.

Or perhaps with Andy seemingly in decline, maybe he should.

Roddick, in New York terms, was king of the hill, top of the heap, when at age 21 he won the Open back in 2003. What a future, we thought. Unfortunately, Roger Federer hijacked that future.

Andy did get to another Open final and three Wimbledon finals, but each time he also got Federer as an opponent.

And now it appears too late. It's not his sport any more. Federer also may be slipping, but Rafael Nadal, Andy Murray and Novak Djokovic, younger and more efficient, have surpassed Roddick. So quickly the years and dreams disappear.

Roddick had his moments. Leinart, the quarterback. hasn't had his, despite entering the pros with considerable publicity and apparent potential.

At USC, Matt had everything, did everything, won two national championships, won a Heisman in 2004.

Some believed he should have entered the draft right there, but Leinart returned for 2005, and even though he was taken in the first round of the '06 draft, it was as the 10th pick, not the first.

Still he was a quick starter for the Arizona Cardinals, after a lengthy rookie holdout, and seemed effective enough. But the second season, 2007, Leinart suffered a broken collarbone.

Fate conspired against Matt. The Old Boy, Kurt Warner was the replacement who through 2008 and 2009 became the fixture, leaving Leinart brooding on the bench.

Warner finally retired after last season, and the assumption was Leinart would take over. After all, he was a million-dollar baby. Instead, it was Derek Anderson, a journeyman. So, angry, Leinart is seeking a trade, which early reports said might be to the New York Giants, if reports the Giants said were nonsense.

How could we be so wrong about Leinart? How could the scouts be so mistaken? Would it have been different if he hadn't been injured?

A first-round pick, a Heisman Trophy winner, ready to be tossed away? Not the way it was supposed to be.

Was it Leinart's attitude? Was it a situation of being in the wrong place at the wrong time? If he goes to the Oakland Raiders or the Buffalo Bills, two teams rumored to be interested, could he resurrect a career which has been a disappointment?

That word, disappointment, could be marginally applied to Roddick. Certainly he has done more than Leinart, but he also has done less than what had been suggested. One Grand Slam?

Was it Federer? Was it Andy's reliance on the serve and inability to master the backhand?

The cognoscenti said 2010 was Roddick's final chance to win that elusive second major, another Open, a Wimbledon. The window in tennis closes all too quickly. One day you're 21, and virtually the next day there's a new 21-year-old coming up from behind who's a step faster.

There was a sadness watching Roddick against Tipsarevic, a sense of inevitability and finality, as if an era, even a lesser one, was coming to a close. If Andy couldn't win this time, why would he be able to win in 2011 when he's another year older?

For Leinart, the opportunity for a breakthrough still exists, if barely. Sometimes a change of teams, and coaches, makes a difference. Sometimes a new system and new teammates are all that is needed.

Or Matt Leinart may be a lifetime backup, good enough to stay in the league but not good enough to be first string, a player who had the potential but little else.

A player to be referenced in terms of what he might have been.

Those, as we've been told, are the saddest of words.