It was another thriller between 14-time Olympic gold medallist Phelps and the man who has emerged as his greatest rival.
Phelps, celebrating his 27th birthday, led by a narrow margin at every turn, but the two swam stroke-for-stroke on the final freestyle lap with Phelps gaining the victory in 1min 54.84sec.
World record-holder Lochte was second in 1:54.93, showing no sign of fatigue half an hour after he secured a berth to defend the 200m backstroke gold he won in Beijing.
In the backstroke, Lochte shrugged off a sluggish start, powering off the last turn and overtaking Tyler Clary to win in 1:54.54.
Clary was second in 1:54.88. Only Japan’s Ryosuke Irie has gone faster than the two Americans this year. Jessica Hardywon in the 100m freestyle in 53.96sec.