Ye Shiwen returned to the Olympic pool after her other-worldly 400 metres individual medley swim to easily head the field in the heats of the shorter medley event.
The 16-year-old produced a mind-boggling final 50m freestyle on Sunday that was quicker than Ryan Lochte, the winner of the men's equivalent.
It saw her cut more than a second off Stephanie Rice's 2008 record and the Chinese swimmer's effort has prompted bafflement.
She was easily quickest in the 200m IM today, in which she is world champion, her time of two minutes 08.90 seconds, 1.61secs ahead of Kirsty Coventry.
Defending champion Rice, who qualified ninth, said: "I really just wanted to get into the semis. I knew I was against Ye Shiwen. She's a great 400 swimmer, she comes home like a gun in that freestyle."
Of Ye's 400IM, the Australian said: "Insane. I mean I didn't see it, I was way over and behind, so I didn't really see her coming home, but that split coming home was out of control, faster than everyone but (Michael) Phelps I think. So if I was up with her I would have had no chance anyways."
Asked whether the teenager's effort is legitimate, Rice added: "I have no idea, I mean I wouldn't want to get into that at all, but a 58 (second final 50m) is an insanely fast swim, but I know she's a good freestyle swimmer.
"I swam next to her at worlds in the 200 IM last year and she came home over the top of me in that freestyle leg and I'm not exactly a bad freestyler, so she's a gun in freestyle."
Defending champion Federica Pellegrini was quickest through in the 200m freestyle in 1:57.16 while Dinko Jukic qualified fastest in the 200m butterfly in 1:54.79 with Phelps back in fifth.