Andy Roddick, who says he will retire after the U.S. Open, celebrates on Sunday after beating Italy's Fabio Fognini in the third round. On Tuesday he faces 2009 champion Juan Martin del Potro of Argentina. (Mike Groll/Associated Press)

Upon hitting a winning drop shot during his second-round victory over Bernard Tomic last week, Andy Roddick erupted in a smile — not simply because he had won the point , but because he had ventured outside his comfort zone. During the same match, he blasted a serve at 139 mph, a speed he hasn’t touched in years.

Two days later, the forehand-slugging baseliner charged the net 66 times against Fabio Fognini in a rare, sustained display of aggression.

To say that Roddick has competed with abandon at the U.S. Open is an understatement. Off the court, the American has found himself grinning, humming and whistling as he strolls around the tournament grounds.

None of this is because Roddick feels destined to win the 2012 U.S. Open. Rather, it’s because he has declared the 2012 U.S. Open the last of his career, announcing Thursday that he’ll retire when his run here comes to an end.

Ever since, it’s as if the 30-year-old Roddick has been reborn. Without the pressure that has taken so many forms since he won the 2003 U.S. Open — the pressure to win a second major, to maintain his top-10 ranking (which has now slid to 22nd), to uphold America’s once-grand tennis tradition — Roddick’s performance has improved.

Asked if he felt the two were related, Roddick said he wasn’t sure. He was just happy, he said, to be playing well in his final U.S. Open, even if he didn’t fully understand why.

But those who have competed against him, as well as those who have studied the effects of pressure and stress on world-class tennis players, see a correlation.

“The guy is uncompromising, and his stubbornness is strength and a flaw,” former touring pro Justin Gimelstob said of Roddick, a longtime friend. “If Andy can’t be the best, or try to be the best, or think he can be the best, he’s not going to stay at something, regardless of what it is. There is no doubt that [announcing his retirement] has given him a positive jump and an adrenaline kick.

“That’s what you’ve seen the last week. He’s playing the right style: Dictating with his forehand; looking to finish at the net; standing in closer to the baseline; playing big, power tennis.”

Even so, Roddick’s career may end Tuesday night, when he faces seventh-seeded Juan Martin del Potro for a spot in the quarterfinals. Roddick’s record against the Argentine is 1-3, though he won their most recent meeting, on a hard court, in 2011.

If Roddick serves well, knocks del Potro out of rhythm by varying the pace, handles the Argentine’s big first serve and leverages the capacity crowd in Arthur Ashe Stadium, he stands a good chance of winning.

It’s all part of Roddick’s playbook.

“I’m comfortable out there,” Roddick said of 24,500-seat Ashe stadium, the biggest venue in tennis, where the outpouring of support the last few days has exceeded anything he imagined. “I’d be an idiot not to use the crowd right now. It’s a huge advantage. Each match is almost like it’s another memory.”

But Roddick’s biggest advantage against the 6-foot-6 del Potro, and for however long his 2012 U.S. Open lasts, may be the stress he’s no longer feeling.

According to sports psychologist Allen Fox, author of “Tennis: Winning the Mental Match,” stress is the biggest challenge professional players confront. The sport is a one-on-one mental and emotional battle, he adds. While most people can handle comparable stress for 20 or 30 minutes, it’s the rare athlete who can endure it for three or four hours.

When a tennis player is gripped by stress, Fox notes, it tends to short-circuit his logic. He pointed to John Isner’s fifth-set collapse against Philipp Kohlschreiber on Monday morning, when Isner smashed a racket, drew a point penalty and proved his own worst enemy, as an example.

“Stress will make you exaggerate your problems,” Fox said. “It distorts your thought-process. It makes you think a bad call is the end of the world. Isner got a bad call, and he thought the world was conspiring against him.”

Roddick has had his share of self-inflicted implosions during his career. But the moment he announced his retirement plans, much of his stress vanished. Add to that the support Roddick has received from the crowd, and it’s no wonder he’s hitting freely.

“All this love and all these good feelings help you as a player,” Fox said.

Veteran tennis coach Vic Braden, who has researched and written extensively about the mental aspects of tennis, agrees. He attributes Roddick’s impressive play at the U.S. Open — Roddick has dropped only one set through three matches — to what he calls a “neuro-scientific reason.”

“When you’re happy, you’re relaxed, and you make better decisions,” Braden said in a telephone interview. “And when you get mad at yourself or frustrated, that goes dark. Andy has made a decision. He’s happy. Win or lose, it doesn’t matter. He’s going out happy. He’s at peace with himself.”