Hannah Miley defied being in the midst of hard training and a heavy race programme to post a world-class time in Leeds on Friday.
Miley was competing in the 400 metres individual medley at the British Gas International Swim Meet 2013.
Fellow Scot Michael Jamieson, the Olympic 200m silver medallist, was due to return to the pool on Saturday morning after posting a win in the 100m breaststroke from lane eight after only squeezing through following a swim-off.
Miley had suffered a rare domestic defeat over the shorter medley courtesy to Siobhan-Marie O'Connor on Thursday.
On Friday she returned for her favoured 400m event and overhauled fellow Olympian Aimee Willmott on breaststroke before pulling away on freestyle for victory in four minutes 35.57 seconds.
While surprised, Miley pointed to the fact she was able to qualify with a far slower heat time than she could in international competition.
Miley said: "The fact I kind of played about and took it quite easy in the morning meant I could go a lot faster - but I didn't expect to go that quick.
"But it's all part of the training and the process that hopefully when I'm rested I can get quicker."
She added: "The fact I can go that time now unrested is a good start but it's not an exact indicator as to how fast I can go in June (World Championship trials).
"We'll see how it goes and hopefully my dad (coach Patrick) will have a few more tricks up his sleeve."
Despite being out in lane eight, Glasgow-born Jamieson held off a fast-finishing Craig Steeples to win the breaststroke in 1min 00.82secs.
The 25-year-old said: "I'm pretty happy with that, especially after this morning with an error of judgement in the heats and I had to do an extra 100.
"This meet is more about the racing than the time - some are rested, some aren't, some are shaved, some aren't.
"It bodes well for the 200."
Jamieson trains at Bath ITC under Dave McNulty, which has become something of a magnet since both he and Andrew Willis made their mark at London 2012, and the pair face off on Saturday.
New Bath team-mate Joe Roebuck added the 200m butterfly title to the 100m he won on Thursday.
The 27-year-old led from the off to touch in 1:57.74 and said of Bath: "I've been training with the university group so it's half the numbers which means you get more attention.
"Dave is a fantastic coach. Training with a high-performance group with such good-calibre athletes, it just rubs off and everyone does well."
Saturday morning's heats were due to feature world 50m backstroke champion Liam Tancock in the 100m event, Fran Halsall in the 50m freestyle in which she was fifth in London with Jemma Lowe going in the 200m butterfly - now shorn of world silver medallist Ellen Gandy, who has switched allegiance to Australia.
