The showdown is on. Hoots and hollers emerged from the morning crowd at the 2012 U.S. Olympic Team Trials in swimming when Michael Phelps walked onto the pool deck Monday morning to compete in the heats of the 400-meter individual medley, yet rival Ryan Lochte laid down the best time entering the night’s final.

Lochte, the world champion in the event, finished his heat in 4 minutes, 10.66 seconds, but he showed plenty of respect for the relaxed 4:14.72 Phelps managed in leading an earlier heat. The pair will face off in the opening final of this week’s trials, with only the top two finishers winning berths for the Summer Games in London.

“That was the easiest 4:14 that he’s ever done, that I’ve ever seen in the entire world,” Lochte said. “It looked really, really smooth for him. Tonight, it’s definitely going to be a dogfight; it’s not just going to be me and Michael . . . it’s also going to be Tyler Clary.”

Phelps’s time was the second-best of the morning, with training partner Chase Kalisz next in 4:15.78. Clary, who finished fourth in this event at the 2008 trials, posted the fourth-best time in the prelims, touching the wall in 4:15.88.

Clary admitted he was surprised that Phelps decided to tackle the 400 medley. Indeed, after declaring that he was done with the grueling event following his world record-performance at the 2008 Summer Games , Phelps has been coy about whether he would swim it here even while testing it out all year.

Phelps admitted with a grin as he hurried off for a warm-down swim that, “I’ve known it for a while, guys. You guys had to know that.”

Bob Bowman, Phelps’s longtime coach, said Phelps made the decision in late March during a grand prix event in Indianapolis. Bowman said he urged his star to conserve energy in the morning swim.

“He and Ryan went out a little differently,” Bowman said. “Ryan strategically did some things fast and Michael did not do anything fast.”

Lochte, who has for months said he expected Phelps to compete in the event, said he also tried to resist the temptation to go out too hard in his heat, holding back over the first 200 meters.

“Usually my first race, I tend to just go for it because I usually feel good,” Lochte said. “So I held back on the first half. . . . I know I have a lot left. I feel good. My legs are still a little sore; as I go throughout this meet, I think I’m going to get faster.”

Phelps’s time was just about a second slower than that he posted at the 2008 U.S. Olympic trials. Lochte called his effort “a really good time for me in the morning.”

“Of course I want to win,” Lochte said. “I hate not winning. I mean, I love to win, but if it doesn’t happen, it doesn’t happen. I’m not going to beat myself up over it.”

Dana Vollmer put up a blazing time in the preliminary round of the 100 fly, finishing in 56.59 and going under the U.S. Open record of 56.64 set in 2000 by Inge de Bruijn. Two-time Olympian Natalie Coughlin finished fourth in 58.41.

“I’ll be less nervous after getting the first swim done,” Vollmer said. “That race felt good, but I didn’t put all I had into it.”