Hannah Miley and Caitlin McClatchey both safely negotiated their way through the heats of their respective events as the third day of action got under way at the Aquatics Centre.
While the pair will race again tonight, there was disappointment for Roberto Pavoni, Sophie Allen, Joe Roebuck and Rebecca Turner, with the latter two both finishing 17th, one spot outside qualification.
Miley was fifth in her favoured 400 metres individual medley on Saturday night and today the 22-year-old returned to post the 10th fastest time of two minutes 12.27 seconds in the 200m.
The 22-year-old Scot said: "I'm really really happy, it's great to make it back into the semi-final
"I always take it one stage at a time and now I'm in the semi-final and the next stage is to make the final. You can't predict how the other girls are going to swim, it's never how everyone predicts but I feel a lot more relaxed after the 400m."
The Garioch swimmer was swimming alongside Ye Shiwen, who swam a faster final length in the 400m medley event than Ryan Lochte, winner of the men's equivalent.
The 16-year-old is the world champion in the shorter event but Miley was unconcerned at being in an adjoining lane.
"Not really because you don't know what to expect from everyone," she said. "Some are going to go out hard at the start, some are going to come back on different legs."
Allen was devastated after finishing 21st in 2:14.72.
"My aim was to make the final so to not even make the semi-final, I'm gutted," she said.
"The time is in line with my fastest heat swims but I need to step up my game in the heats.
"I've always struggled to get my arse in gear in the mornings, but that's no excuse when you come to a Games like this."
McClatchey eased into the 200m freestyle semi-finals in joint seventh but there was disappointment for Turner who missed out by just two hundredths of a second.
McClatchey is competing at her third Games having reached at least one final on each occasion, including a sixth-place finish in this event in Beijing.
The 26-year-old swam the anchor leg for the sprint relay that came fifth earlier this week and today she looked impressive with her customary smooth technique to win her heat in 1:58.03.
McClatchey said: "I had great confidence from the 4x100 because I was on form so I was hoping I could do a good job this morning and to win my heat against some of the best girls in the world was fantastic.
"Obviously they probably were a bit tired from the 400 last night, but who cares?"
The Loughborough swimmer was inspired by Rebecca Adlington's bronze medal in the 400m freestyle last night while tonight her boyfriend Liam Tancock will go in the 100m backstroke final with Gemma Spofforth in the women's equivalent.
She added: "I took massive inspiration from that and used the crowd this morning."
Of Tancock's chances, she said: "I really hope he's going to do well. I don't know if im going to be able to watch the whole race without my hands covering my eyes but I am really looking forward to it and hope he can get a medal."
Turner was in the same heat as defending champion Federica Pellegrini who, as in Beijing, finished fifth in the 400m.
The Italian impressed this morning to lead the way in 1:57.16, 0.17secs ahead of Allison Schmitt, who set a textile world record at the US trials last month and who was second over 400m yesterday.
Turner, though, was always off the pace and her time of 1:58.98 was more than 1.3secs off the personal best she set in this pool earlier this year.
It was only good enough for 17th as she was edged out by two hundredths of a second by Beijing silver medallist Sara Isakovic.
The City of Sheffield swimmer said: "I don't know where it went wrong, we have all been training so hard. It was just not my day."