Michael Phelps used his powerful shoulders to perfection to overhaul big rival Milorad Cavic and steal back his 100 metres butterfly world record in an enthralling final at the world championships on Saturday.
The Olympic great was trailing in fourth after 50 metres with Serbia's Cavic leading the pack but the roars of the Roman fans spurred the American on to retain his title in style.
His long arms reached out to touch the wall first in 49.82 seconds, beating the 50.01 Cavic had laid down as a marker in Friday's semi-finals.
Phelps lost his big battle with Germany's Paul Biedermann in the 200 freestyle final on Tuesday but rose to the occasion against Cavic, who took 50 butterfly gold on Monday.
A major rivalry between the pair is now burgeoning.
"It's almost literally like a straight showdown, like boxers going face to face at the weigh in," Phelps said.
"It's kind of cool for the sport. I have a feeling this battle is going to go back and forth for a while."
Cavic, wearing a soon-to-be banned Arena suit deemed quicker than Phelps' Speedo, added: "When I hit the 50 he was much closer than I expected.
"Michael Phelps is Michael Phelps. He does what he does and he did."
The victory gave Phelps his fourth gold of the meet following triumphs in the 200 butterfly, the 4x100 freestyle relay and the 4x200 freestyle relay, with the defeat by Biedermann the only blemish in a scaled-down programme.
He chose to compete in fewer races than normal in the Foro Italico open pool after only just returning from a six-month break at the U.S. trials in Indianapolis last month, where he set a previous 100 butterfly record.
After 35 suit-aided world records in the previous six days, and a morning trip to visit Pope Benedict for some top swimmers, Saturday's session started slowly with Australia's Marieke Guehrer causing a surprise by winning the women's 50 butterfly.
Brazil's Cesar Cielo Filho then completed a freestyle double by adding the 50 title to the 100 crown he won on Thursday, but the lack of world records was puzzling the spoilt crowd.
Zimbabwe's Kirsty Coventry duly delivered by gliding home in two minutes 04.81 to seize gold in the 200 backstroke.
Sunday's men's 50 backstroke final looks wide open after Britain's Liam Tancock unexpectedly produced a world record of 24.08 in the first semi-final to fire a warning to Japan's Junya Koga.
Britain had less to cheer in the women's 800 freestyle final when Dane Lotte Friis beat Joanne Jackson to gold with Olympic champion Rebecca Adlington only managing fourth.
The day's programme ended in a flourish with China smashing the world record to win the women's 4x100 medley relay.
Australia came second but there was slight consolation for relay member Sarah Katsoulis, who qualified quickest for the 50 breaststroke final on the last day of competition on Sunday.
Compatriot Cate Campbell was also fastest in 50 freestyle qualifying.