At the 2008 Summer Games in Beijing, Michael Phelps wasn’t just dominant, he was unbeatable. And he wasn’t just unbeatable, he was incredible, setting world records in each of the eight events in which he won gold medals.

This year, things promise to be much more challenging right from the start. Phelps will be tested by U.S. teammate Ryan Lochte, who is favored on Day 1 of the swimming competition Saturday to hand Phelps the defeat he never experienced in Beijing.

With Phelps chasing as many as seven golds in these Games, and Lochte as many as six, their encounter in the 400-meter individual medley could set the tone for the entire competition. The winner of Saturday’s showdown will take the lead in the unofficial race to determine the world’s top male swimmer.

“They’re fun, exciting races, and Ryan has proved that he’s been the best over the last couple of years,” Phelps said. “For me, I’ve always been about being able to race the best.”

These Games represent a chance for Phelps, who has said he will retire after the London Games, to put an exclamation point on his greatness.

Three more medals would give him 19, more than any athlete has ever won in the Olympic Games. Lochte, on the other hand, has a chance to push Phelps off of his throne, rather than awaiting his departure. He will also face Phelps in the 200 individual medley.

At last year’s world championships, Lochte hinted that he was ready for the role of usurper. He defeated Phelps in two races, the 200 individual medley and 200 freestyle, and won more gold medals overall. At the recent Olympic trials, however, Phelps regained the mental edge, defeating Lochte in three of four head-to-head finals.