Australia's swimmers were served an early reminder that paper rankings will count for little at the world championships after David McKeon and Bronte Barratt were surprise casualties on day one.

Despite both being ranked second in the world coming into the event in Barcelona, McKeon and Barratt fell well short of booking spots in the men's and women's 400m freestyle finals on Sunday.

They posted times more than five seconds slower than their efforts in winning national titles in April and promising talent McKeon was particularly shattered and puzzled by his swim.

"Pretty s***. I just don't know what happened," said McKeon, the son of former Olympian Ron McKeon.

The 20-year-old faded late as he clocked three minutes and 49.51 seconds to be 12th fastest in the heats but 17-year-old compatriot Jordan Harrison had no such problems as he impressed to qualify third fastest for Sunday night's final.

Harrison, who is also competing in the 1500m and 800m freestyle events, posted 3:46.85 to progress behind Chinese superstar Sun Yang 3:44.67 and Canadian Ryan Cochrane (3:45.74).

Barratt said she simply had no legs as she swam a 4:09.65 to be 13th fastest behind American pacesetter Katie Ledecky (4:03.05) in the women's heats.

Australian Kylie Palmer booked a spot in the final, posting 4:05.01 to be fourth fastest.

Barratt was more upbeat than McKeon, with most of her training aimed at the 200m freestyle later in the week.

"It was pretty tough, it was pretty terrible," Barratt said.

"I just had no legs in the last 100.

"I think the first swim's always really tough but that's alright, I've got the 200 and I'm looking forward to that."

Harrison said he tried not to watch McKeon's race before contesting the final heat and he was delighted to progress into his first major international final.

"It was pretty overwhelming," said Harrison.

"I didn't really think much during the race. I was just trying to stick with Sun Yang and Ryan next to me. I was kind of just going for it.

"To come out third fastest, that's really good. It's been a really great experience so far."

Australian star Alicia Coutts said the first session of heats had shown world rankings meant nothing when it came to big meets.

"I don't really pay too much attention to paper rankings, you just never know what's going to happen," Coutts said after progressing to the 100m butterfly and 200m individual medley semis with minimal fuss.

Veteran Brenton Rickard also dropped out in the heats of the 100m breaststroke, an event in which he is a former world champion.

Richard was 17th-fastest in 1:00.52 to miss the semi-finals while compatriot Christian Sprenger was the top qualifier in 59.53.

There were no surprises as Australia's men's and women's sprint relay teams secured their spots in Sunday night's 4x100m finals.

A team of James Roberts, Kenneth To, Matt Targett and Tommaso D'Orsogna clocked 3:13.04 to be third fastest into the men's final behind the impressive US (3:11.69) and Russia (3:12.43).

Individual 100m swimmers James Magnussen and Cameron McEvoy were saved for the final, with To pushing his claims for a berth in the decider with the quickest heat leg for Australia.

Australia's Olympic champion women (3:36.46) were second quickest behind the US (3:36.22), with Emma McKeon particularly impressive in a team missing big guns Cate Campbell and Coutts.

Emily Seebohm backed up for the relay after earlier qualifying third fastest for the 200m individual medley semis in 2:11.12, while Coutts (2:11.88) seventh-fastest qualifier.

Coutts was fourth quickest into the 100m fly semis in 57.56 behind Olympic champion Dana Vollmer (57.22) and Australian Brittany Elmslie (58.27) was through in eighth.

"I tried to make them as relaxed as possible," Coutts said of her morning swims.

Targett was ninth-fastest through to the semi-finals of the non-Olympic 50m butterfly.