Ryan Lochte set the pace in the men's 200m freestyle Monday as the United States looked to recover strongly from their stinging world championships defeat in the 4x100m relay.
"I didn't expect it to hurt that much," said the three-time Olympic gold medallist, ahead of the semi-finals later on Monday.
"It was pretty good swim for a morning swim for me. Tonight I think I'll have Michael next to me so it's definitely going to be a good race."
France's European champion Camille Lacourt topped the men's 100m backstroke standings with Britain's world 50m title-holder Liam Tancock fourth. Denmark's Lotte Friis was fastest in the women's 1500m heats.
In Monday's evening finals, Jessica Schipper carries Australian hopes in the women's 100m butterfly, a day after her team-mates buried the United States' seven-year world and Olympic 4x100m freestyle men's relay streak.
Controversial Brazilian Cesar Cielo is favourite in the men's 50m butterfly, just days after he was cleared to compete despite testing positive for a banned substance.
Source: AFP American Edition