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 set a new mark.
The race had been expected to be a shootout between the two but Arlen's participation was thrown into doubt earlier this week when she was deemed ineligible for competition and excluded from London 2012. The United States' appeal to the International Paralympic Committee was upheld and the 17-year-old was reinstated on Thursday.
It is a two-stage process with Arlen having passed an initial assessment but she was also observed during racing on Saturday after which a final decision was set to be reached.
The IPC announced on Saturday morning they would make a statement as to her fate on Saturday afternoon, before the final into which she was the second fastest qualifier behind Simmonds.
The situation clearly had not affected Simmonds who 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.
Simmonds 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."
Of the circumstances surrounding her fellow teenager Arlen, who clocked 5mins 25.19secs, Simmonds said: "I've been aware of it, I've just had to focus on myself, I can't control it. At the start of the week she was an S6 and now she is an S6."
While Simmonds will be the most high-profile Briton in the pool on Saturday night, Matt Whorwood is also in with a medal shout after he qualified third in the men's S6 400m freestyle. The Truro-born swimmer won bronze in Beijing four years ago and his time of 5:17.28 - more than 11 seconds clear of his closest rival - indicates he is on course to at least match that.