Steadiness, every bit as much as a booming serve, has been the trademark of Andy Roddick’s career. Since 2001, his second year as a professional, he has reached at least one quarterfinal of a Grand Slam each season. He has ended the year inside the top 10 every season since 2002, including a finish at no. 1 in 2003 — the year he won his only major title, at the U.S. Open. Though Roddick hasn’t won a major title since, it’s only because a modest talent by the name of Roger Federer — he of the 12 major titles and recently ended streak of four-and-a-half years at no. 1 — has blocked Roddick’s path. Federer has beaten Roddick in three major finals and one major semifinal. Imagine how different Roddick’s career would look if Federer had not been Federer for so long.

At this year’s U.S. Open, however, Roddick, whose 26th birthday is this week, finds himself on unusually shaky ground. He has seen more ups and downs this season than in any other in his career, more nagging injuries, and more disappointing results. (He has yet to reach the quarterfinal of a major, putting his streak in jeopardy.) This summer, Roddick had hoped for an extended warm-up leading up to the U.S. Open, a waltz through two American tournaments while his chief competitors tired themselves out at the Beijing Olympics. Instead, Roddick struggled, largely because a recent injury to his right shoulder has limited his practice time. In Los Angeles and in Washington, D.C., he lost two matches to two talented youngsters whom few Americans could identify in a lineup of tennis players: Juan Martin Del Potro, a 19-year-old Argentine, and Victor Troicki, a 22-year-old from Serbia. Roddick was the top seed at both tournaments.

Perhaps some American fans are wondering, “Is this the beginning of the end for Andy Roddick?” The answer is no, since big servers tend to remain dangerous at older ages as long as their shoulders stay healthy. (Goran Ivanisevic, you’ll recall, won his first and only Wimbledon title at age 29, and Pete Sampras served his way to titles into his 30s.) As John McEnroe put it last week, Roddick will always have a “puncher’s chance” at every tournament played on a fast surface. Remember, too, that only a few months ago, Roddick was playing some of the best tennis he has played in years, if not ever. In Dubai earlier this year, he beat Rafael Nadal and Novak Djokovic, each in straight sets, on consecutive days. Not long thereafter he beat Federer for the first time since 2003, a victory that ended an 11-match losing streak. He has yet to regain that form since hurting his shoulder.

That said, I’d be surprised if Roddick has as many chances to land haymakers in the next few years as he once did. In Roddick’s nine years on tour, his best weapon — one of the best serves the sport has ever known — has become less important than it used to be.

It’s easy to forget how novel Roddick’s serve, and its abbreviated, violent delivery, was when he turned pro in 2000. Back then, Roddick was the Joba Chamberlain of tennis — a tall, powerful 18-year-old from Nebraska with a fastball like no one else in the sport. His serve regularly approached 140 mph, unthinkable in tennis at that time. (In 2004, he set the record that stands today, 155 mph.) Roddick finished his first season ranked no. 158 in the world, but it was clear he would eventually become one of the game’s best players.

Great things happened more quickly than anyone had expected. In 2001, Roddick reached his first major quarterfinal. In 2003, he blossomed, reaching the semifinals of the Australian Open and Wimbledon and winning the U.S. Open. He finished the season as the no. 1 player in the world, eclipsing Roger Federer, who was a year older and wildly talented, yet still seen as something of an underachiever. (Federer won his first major title, at Wimbledon, in 2003.) Through those years, Roddick confounded opponents with his booming serve and with second serves that had a lot more spin, and as much speed, as many first serves.

These days, no one serves as hard as Roddick, but many players come close. (If you’re going to the Open this week, watch Del Potro, Djokovic, Gael Monfils, Jo-Wilfried Tsonga, Ivo Karlovic, and many others.) A tour with more booming serves does not favor servers — no, it helps returners, who see these serves all the time and learn to cope with them. If a lot of players can serve exceptionally well, then something else is going to decide a match. Today, the best movers, defenders, and creators of angles reap the biggest rewards. Federer began to dominate when he began to play with less abandon and emphasized defense and reactive shot making; Djokovic also does this well. (The Serb’s lateral movement skills are first rate.) Nadal, of course, has redefined the art of defense. He retrieves almost every shot sent his way and can attack from terrible positions on the court.

Roddick has never had the strokes, speed, and quickness of Federer and Nadal — and not for lack of effort, either. (Roddick trains vigorously.) For a time, his serve was enough to carry him consistently. Now, it’s enough to carry him on any given day, but not as often as it once did. Roddick’s big serving style was once thought to be the future of tennis (to the dismay of many). It’s now the past: A big serve, on its own, can’t win enough matches. The rest of the field has adjusted, so much so that it’s now Roddick’s turn to adapt to them or his other stock in trade, consistently good results, might be a thing of the past, too.

Mr. Perrotta is a senior editor at Tennis magazine. He can be reached at tperrotta@tennismagazine.com.