A year to the day since his Olympic triumph, Wiffen came out of the blocks fast and led the race after the opening 100 metres, when he was on world record pace from the outside lane.

He maintained his advantage at the 300m mark, but was soon caught and passed by Jaouadi, Schwatrz, Martens and his American Olympic rival Bobby Finke.

The Ireland swimmer continued to fall away as Jaouardi stretched his margin out front, and Wiffen would eventually come home in eighth place – some 22 seconds behind the Tunisian.

Jaouadi’s time was the third-fastest ever in the discipline after a blistering final 400m.

Despite a disappointing defence. of his 800m crown, Wiffen will try to reclaim his 1500m freestyle title when the heats start on Saturday.

Elsewhere, Ellen Walshe produced a superb swim in her 200m butterfly semi-final to progress to Thursday’s final with the fourth fastest time overall.

The 23-year-old posted a time of 2:07.48 to finish third in a quick second semi-final behind Summer McIntosh and the USA’s Regan Smith.

Danielle Hill posted a season’s best time in the 50m backstroke but missed out on the final by setting the 12th fastest time in the semi-finals.

Hill posted a time of 27.71 seconds and said she had “very mixed emotions” after her swim.

“I want to be in that final and I’ve been around that time for a couple of years, so there is something we have to change in order to improve,” she said.

“I’m disappointed not to be in that final but it’s a massive learning curve this week.”

Hill is back in the pool in the 100m freestyle on Thursday morning, along with 200m breaststroke swimmers Ellie McCartney and Mona McSharry.