For the second time in a day Simmonds broke her own world record, taking more than a second off the mark set in qualifying to win in 3min 05.36sec. Germany's Verena Schott took silver, a barely believable nine seconds behind, while Great Britain's Natalie Jones picked up bronze after a strong first half of the race.
After the close-fought drama of Simmonds' victory over the USA's Victoria Arlen in the 400m freestyle, this was a far more dominant performance. She kept in touch with the leader, Ukraine's Oksana Khrul, throughout, relying on her powerhouse freestyle finish. The final leg was a tour de force as Simmonds ate up a three-metre deficit at the turn to leave the field for dead.
This is by no means Simmonds's favourite event but it is the one for which by her own admission she has trained hardest in the four years since Beijing. Backstroke is not her strongest suit, though she swam it strongly here. And she concedes considerable advantage to her taller competitors over the breaststroke and butterfly legs, where sinuous lower-body movement propels the swimmer. But at freestyle she is peerless and her rapid, aggressive stroke, like a piranha that has scented blood, rapidly makes up the difference.
Earlier Simmonds had laid down a marker in taking more than a second off her own world record to qualify fastest in 3.06.97. For an event in which she could finish only fifth at the previous Paralympics it provided further evidence that here is an athlete maturing to the peak of her abilities.
The 17-year-old, who won two golds at the Beijing Games, can now go at least one better.
Further duels against Arlen await in Tuesday's S6 50m freestyle final and in the 100m on Saturday, with Simmonds still on course for a remarkable four-medal haul.
guardian.co.uk © Guardian News and Media Limited 2010
image: © Fighting Irish 1977