Wiffen, who is the European record holder in the event in 7:39.19, led at the halfway mark of the race but with 100m remaining there were still five swimmers vying for the win.
The 22 year-old produced a sprint finish in the closing stages to touch the wall 0.38 of a second De Tullio.
“I knew it was going to be tight, I knew exactly what pace we were going, so I was like I don’t want to leave it to chance coming off in the heat, so top 2 is what we wanted and see if we get through.”
Looking ahead to the final, Wiffen added: “Yeah I’m looking forward to it, getting into my preferred schedule now, just 800 and 1500 and yeah as I said looking forward to it, going to be a great race.”
Victoria Catterson swam under the two-minute mark for the third time in her career in the 200m freestyle heats.
Ards native Catterson, who holds the Irish record at 1:59.74, was right at her best, touching sixth in her heat in 1:59.75. The Ards swimmer finished nineteenth overall
Darragh Greene was 18th overall in the 50m breaststroke clocking 27.76, missing a semi-final place by just 0.14 of a second.