Michael Phelps after his victory on Saturday. (Reuters)

Rome: Michael Phelps beat Milorad Cavic again, and this time there was no doubt about it.

With a defiant performance in a supposedly inferior suit, Phelps stayed close over the outward lap and rallied on the return to become the first swimmer to break 50 seconds in the 100-metre butterfly, beating the Serbian with a time of 49.82.

Cavic also broke 50 seconds, but 49.95 was only good enough for silver. He wasn’t nearly as close as last year’s Beijing Olympics, when Phelps famously won by one-hundredth of a second and Cavic always maintained that he actually touched first.

Britain’s Liam Tancock has set the 38th world record of the world swimming championships in the men’s 50-metre backstroke.

Tancock posted a time of 24.08 in a semi-final heat Saturday night, beating the mark of 24.33 held by American Randall Bal. Junya Koga of Japan also went under the previous record time to win the second heat at 24.29.

When Phelps saw his time at the Foro Italico, he hopped up on the lane rope separating him and Cavic, threw up his arms and let out a scream toward the US cheering section.

Showing as much emotion as he ever has, Phelps also slapped the water and tugged at his Speedo LZR Racer swimsuit — no doubt in reference to Cavic offering to get Phelps one of those supposedly faster polyurethane suits so he wouldn’t have any excuses.

Also on Saturday, China smashed the world record to win the women’s 4x100 medley relay, clocking three minutes 52.19. Australia was second and Germany took bronze.

In the women’s 200 metres backstroke final Kirsty Coventry seized gold with a world record. The Zimbabwean swam two minutes 04.81 to beat her previous best mark of 2:05.24 which she set in winning gold at last year’s Beijing Olympics.