London - The eight-day Olympic swimming programme starts with a bang on Saturday with US star Michael Phelps and his arch rival Ryan Lochte on a collision course in the men's 400m individual medley.
Phelps' haul of eight gold medals from the Beijing Olympics four years ago included the 400m medley title, but the 27-year-old swim legend was beaten by Lochte, the reigning world champion, at the US trials three weeks ago.
Competition at the Aquatics Centre kicks off Saturday morning with the men's medley heats, and the final is the first race of the evening session.
Phelps is aiming to become the first male swimmer to win a gold medal in the same individual event in three successive Olympic Games and will not surrender his crown without a fight.
"It's going to be a very challenging and exciting race, I have changed a few small things to see if it will help (since the US trials)," said Phelps, who has amassed 14 Olympic gold medals.
"I am feeling good, I am feeling confident. It is going to be a fun one to start. You can guarantee it will be loud on opening night," said Phelps.
Most interest centres on whether Phelps can repeat his golden Beijing form against the in-form Lochte, who took the 400m medley gold at last year's world championships in Shanghai, an event Phelps sat out.
"For anyone who wants to promote swimming, there is not a better way in the world than for (Michael Phelps) to swim the first race in the first (swim) event at this Olympic games," said his coach Bob Bowman.
"It will be a coaches' and spectators' dream to see how that shakes out."
Aside from the American duo, Hungary's Laszlo Cech and Brazil's Thiago Pereira are others to look out for, but Lochte and Phelps have posted times nearly two seconds faster than their rivals so far this year.
Lochte, however, insists it is far from a two-man race.
"I'm not just swimming to beat Michael Phelps, there are a bunch of swimmers from across the world who I have to worry about," said the 27-year-old.
"I'm the defending (world) champion from last year, but that was last year.
"I was the world's best swimmer, everyone said at the time, but I put myself down at the very bottom so that I have a goal to put myself back up to the very top.
"If Michael is right there with me, then all well and good, but I can't rely on just one person.
"We have a great rivalry, but at the same time we have created a great friendship and no matter win or lose, we will be friends after the race."
With eight lengths of the 50m-long pool to be swum twice in the same day over the four strokes, with the heats and the final, Phelps said energy conservation will be a factor in the first of his seven events in London.
"You have to... I don't want to say hold back... but you have to put yourself in a good position, you have to weigh up how much energy it will take to get into the final," he admitted.
"You can't dump the whole thing in the heats, I have a long meet and a lot of swims, so I have to pace myself."