But there was only one competitor that everyone was watching and talking about in suburban Phoenix on Thursday.
Michael Phelps has always been the centre of attention in swimming circles since he burst on to the scene as a teenager and his comeback at the USA Swimming Grand Prix meet had everyone in a lather.
The spectators roared when he won his morning heat then gasped when he lost the 100 metres butterfly to his old rival Ryan Lochte, who was as happy as anyone to see him back.
"Just our history, between me and Phelps, any race we go up against each other it's going to be a battle because we don't know how to give up," said Lochte, who has five Olympic gold medals.
"The thing that we do the best is we race, we know how to get our hand on the wall first. It doesn't matter if it's breaststroke, a mile, or 100 fly, we're going to go up there and race tough.
"One thing being a racer and being in the Olympics and swimming all those events, one thing you don't lose is the racing ability, that competitive edge.
"He could have taken 10 years off ... but he's a racer and that instinct will be with him until the day he dies."
Lochte's former coach Gregg Troy, who helped mastermind one of Phelps' Olympic defeats, has no doubt the world's most decorated swimmer could be as successful in his comeback as he was before he retired.
"He's the best. It's that simple," Troy told Reuters.
"If he fully commits to doing it, he can do anything he decides he wants to do but that's up to Michael to decide."
Few people know Phelps' capabilities better than Troy, who has served as the United States men's national head coach at various major meets over the past decade, including the 2012 London Olympics.
Troy coached Lochte to the gold medal in the 400m individual medley at the 2012 London Olympics but has also worked in tandem with Phelps and his coach Bob Bowman and seen the pair develop into the most successful partnership in the sport.
"He's capable of doing anything he wants to," Troy said.
"He's actually at the prime age for males, he's not over the hill by any means, it just depends on how he commits his time and what he and Bob decide what to do. They're a great team, they'll get it figured out."
Phelps is currently 28 and would be 31 when the 2016 Rio Olympics rolls around. Phelps has still not said whether he wants to go to the next Olympics but Troy felt it was not beyond him as long as he was motivated to do the training.
"Michael can get to Rio If he's committed to the sport and enjoying it," Troy said. "But it's up to him to decide, he might find he doesn't want to.
"Michael accepts challenges very well so he and Bob will probably come up with some sort of programme that has some challenges and when they do that I think they'll be fine."
Natalie Coughlin, who has three Olympic gold medals, said she was also pleased to see Phelps back competing.
NOTHING TO PROVE
"Everyone has their opinions about why he's coming back but he's so young and he's enjoying it so why not?" Coughlin said.
"He has nothing to prove to anyone. He's the greatest of all time and will always be that so this is just all icing on the cake.
"This is fun, he's enjoying that and I think he understands that but being a competitor he'll still want to be good, you can't get that out your system."
American teenager Katie Ledecky, who won the 800m freestyle gold at London and matched the fastest time in the world this year on Thursday, was also gushing about the return of Phelps.
"It definitely adds some extra energy to the meet and it was great to see so many people here, excited about it," she said.
"I know all the swimmers are all excited about it so it's fun to see him back and we all welcome him back."
- Sports & Recreation
- Swimming & Diving
- Michael Phelps
- Ryan Lochte