NEW YORK (AP) – Andy Roddick walked toward the chair umpire, shaking his head and waving his arms.

“Challenge it, Andy!” shouted a man high up in the stands at Arthur Ashe Stadium.

Instant replay made its debut at Grand Slam tennis Monday and two points from an overwhelming win at the U.S. Open, Roddick wanted to test the system.

Roddick was certain his serve had nicked the net. He’ll never know for sure – only line calls can be reviewed.

“I tried to challenge a let today, but they wouldn’t let me,” the former Open champion said after overpowering Florent Serra 6-2, 6-1, 6-3.

Andre Agassi, however, fared better. He won a key point in his comeback against Andrei Pavel, correctly claiming his serve was good as the crowd roared in agreement.

There were 11 challenges overall on the opening day at Flushing Meadows, three resulting in reversals.

The NFL, NBA and NHL already employ replay, and tennis decided to try it after testing the Hawk-Eye technology at 10 events that led up to the Open. In those tournaments, 39 percent of the 839 challenged calls were overturned.

Mardy Fish became the answer to a trivia question: Who was the first player to ask for a replay at a major?

Fish questioned a sideline call during his win over Simon Greul. A few seconds later, an animated replay appeared on the scoreboard at Louis Armstrong Stadium – the judge’s call stood, Greul’s shot was good.

“It was close. I figured it was in,” Fish said. “I felt like I still had two challenges. Might as well use it.”

Asked how it felt to be the first, he laughed and said, “I have no idea.”

“I like everything about it,” Roddick said. “Personally, for me, I think it’s a good thing because I spend less time whining about calls.”

Players get two challenges per set, plus another one for tiebreakers. If the call is reversed, the player keeps that challenge.

There hasn’t been a lot of opposition, maybe because the system is so accurate. The 10 small cameras tracking shots at both Ashe and Armstrong measure marks within 3 millimeters, an error margin roughly equal to the width of a ball’s fuzz.

Not bad, considering two cameras are mounted almost 115 feet above the blue courts and the others are 281/2 feet off the ground. The two show courts are the only ones at the Open that have replay, at a total cost of about $300,000.

“The human aspect of line calls is gone, obviously,” Fish said. “In a way, it’s good because you’re not going to lose over a bad call.”

And, the results come fast. Too fast, in fact. Tennis officials actually told the Hawk-Eye folks to slow it down a bit so the drama can build in the crowd.

So instead of posting a 3-D, computer-generated image on the stadium scoreboard in two seconds, the virtual replay appears on the scoreboard – with “In” or “Out” – in about eight seconds, just enough time for fans to shout their opinions.

Fans had mixed reactions to the system.

“I don’t like it,” Rich Lee said. “It’s bad, it’s going to slow down the game. And bad calls are part of the game.”

Walking with him near the food court, Pat MacIsaac disagreed.

“I think it takes a lot of the frustration away,” she said. “They ought to just have John McEnroe standing there, making the calls.”

Serena Williams sure wanted to appeal after an obvious call went against her in a key match against Jennifer Capriati in the 2004 U.S. Open. With balls flying around at well over 100 mph and a system that seems to work, tennis decided to try it.

“I think it’s great for the fans,” Roddick said. “I think it adds another dimension to watching it on TV.”

And it gives him a second choice on close calls. Which he intends to use, no matter the score.

“If I think they’re wrong, I’ll challenge every time,” he said. “That’s what it’s there for. I don’t see why you would change it. You’re still trying to win every point, right?”

AP-ES-08-29-06 0032EDT