At the starting dock of the Olympic triathlon trials, the expression on Andy Potts's face seems to say I kill you with my eyes. As the starting gun fires, he plunges into the Black Warrior River in Tuscaloosa, Alabama, and, in a burst of white foam, quickly pulls ahead of nine rivals. The second-ranked Hunter Kemper manages to hold pace with Potts for a few minutes, then drifts back into third place.

Potts's lead grows relentlessly to five body lengths as the rest of the field fans out behind him. He should dominate this Olympic-distance race - a 0.9-mile swim, 24.8-mile bike ride and 6.2-mile run - just as he dominated last year's national championships and Pan American Games. He is, after all, the number-one-ranked triathlete in the U.S. Within minutes, he extends his lead to 30 lengths and swims for the shore alone.

His coach, Mike Doane, paces along the river's edge. "When he can get his heart rate up around 165, he has a great race," Doane says. Any higher than 165 beats per minute, and he's using too much energy too early. Much lower - say, below 140 on the swim - and it means that he's too tired to generate the tempo that would get his heart rate up.

Potts leaps out of the water, charges toward his bike, and zooms onto the cycling course 38 seconds ahead of Kemper. His heart rate, monitored by a microcomputer on his wrist, is right where it's supposed to be: 165. As Potts speeds by, Doane yells to him, "Forty-five-second lead!" Kemper and three others whip by in a thick pack. Potts zips past to begin the second of eight three-mile loops. "Thirty seconds!" The third time: "Twenty-five seconds!" They're gaining on him.