Michael Phelps again outshone Milorad Cavic as the pair renewed their rivalry in the 100 metres butterfly.
Cavic will always be convinced he won the Olympic title in Beijing but the American secured the gold by one one hundredth of a second.
The following year, the Serbian goaded his rival at the World Championships, offering to buy him a 100% polyurethane suit at the height of the shiny suit madness.
That was akin to poking a bear with a stick, Phelps completing a memorable victory in world-record time as Cavic was forced to accept second best.
In today's heats, the pair were alongside each other and again Cavic had a storming start and Phelps appeared to be out of touch.
Last at the turn, the Baltimore swimmer then produced his customary storming second 50m to once again overtake Cavic and touch first in 51.72 seconds, second overall behind his conqueror in the 200m, Chad le Clos, who clocked 51.54secs.
Phelps said: "We have had some pretty closes races in our day.
"Someone tweeted this morning we were next to each other, in the morning swims I haven't been able to catch a break.
"I knew today was going to be a fast one, this is my last prelim swim ever, so it was fun to be able to do a fast time."
The 27-year-old also described the moment he received a phone call from US President Barack Obama yesterday.
"The craziest thing was just when I answered the phone and they were like, "Michael?", and I said "Yes". They said, 'Please hold for the President of the United States', and I was like, 'Okay!'.
"He just got on and was saying how everyone is supporting me and is behind me at home and how proud everyone is of me. That was pretty cool.
"And he finished by saying, 'Make sure you tell your mom I said hi' - it was a good call."
Rebecca Adlington swam the fifth fastest time in the world this year as she qualified quickest for the 800m freestyle final in eight minutes 21.78secs.
George Bovell of Trinidad heads the 50m freestyle heats while Missy Franklin's 2:07.54 leads the 200m backstroke.
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