Ellie Simmonds was narrowly outside the world record as she made her intentions clear in the S6 400 metres freestyle heat where she booked a probable final showdown with American Victoria Arlen.
The British swimmer is the defending champion in the event and had been the world record holder until June when Arlen, who was the second fastest qualifier here, set a new mark.
Simmonds was beaming after her time of five minutes 24.64 seconds, a Paralympic and European record and personal best, just 0.18secs outside Arlen's world mark.
She said: "Really pleased. I am shocked by it.
"All my coach (Billy Pye) said leading into it was to go in there and hopefully do a 5:30. But, yeah, really happy - I've just done a PB in a heat swim which gets me ready for tonight.
"It's going to be very tough but very exciting race but I am looking forward to it."