World champion James Magnussen said it would take one of the year's fastest times to win the Pan Pacific Championships 100m freestyle after American stars Michael Phelps and Nathan Adrian threw down the gauntlet Friday.
Adrian, who nudged out Magnussen by one-hundredth of a second to win the Olympic 100m gold in London, topped Friday's qualifiers with 48.05 seconds.
Australia's two-time world champion was just 0.2sec behind, with Olympic great Phelps surging into contention on the back of 48.45, just four-hundredths of a second ahead of another Australian, Cameron McEvoy.
Phelps, four months into his comeback from retirement, produced a sizzling swim to ensure he would be the second American in Friday's final ahead of team-mates Anthony Ervin and Jimmy Feigen.
Magnussen, who has been troubled by a degenerative back condition, was relishing the twin US threat.
"It feels fine, I'm happy to race anybody on my day so I'll look forward to the challenge," he said.
"It will take a 47 (seconds) to win it, so I think there will be a few of us under that 48-sec barrier."
Only Magnussen and McEvoy have gone under 47 seconds this year, with Brazilian Cesar Cielo holding the world record at 46.91sec.
Phelps, 29, the 18-time Olympic gold medallist, looks to be returning to form after failing to win at this month's US Championships.
"(Coach) Bob Bowman and I have been working just on the stroke and today my stroke actually felt easier than it normally had," Phelps said.
"The first 50 felt really good and I knew I had to win that heat if I wanted a chance to swim in the final tonight. I actually hit the turn which is a good thing."
Adrian said the quality of the field ensured no risks could be taken chasing a spot in the final.
"The field was like a men's team meeting in the ready room and the depth we have is just incredible, it's kind of like the US nationals," he said.
"The times (at the nationals) those guys put up weren't equal to the quality of swimmers that they were, so you couldn't risk anything today."
- Campbell imperious -
Australia's world champion Cate Campbell was imperious in cruising to the fastest time in the women's 100m freestyle heats.
The newly crowned Commonwealth Games champion powered to a meet record and the year's best time of 52.62sec in coasting to victory, 0.88sec ahead of her sister Bronte with American star Missy Franklin clocking 53.65sec.
Campbell said she was not reacting to Sweden's Sarah Sjostrom's time of 52.67sec in winning the European crown in Berlin this week.
"That's my fastest time since the (2013) worlds where I was in the form of my life so to be able to come out and do that I'm very happy," she said.
"I don't keep track of other people, I don't think it's healthy to rate yourself off other people. I can only be as good as I can be.
"I'm sure Sarah and I will have some great head-to-head battles in the years to come."
Franklin was looking forward to an easier day after suffering a back spasm leading into the Gold Coast meet.
"I'm pretty happy with that time, it's really close to where I was at in the prelims at the nationals," Franklin said.
American Kevin Cordes was the fastest qualifier in 59.70sec for the men's 100m breaststroke final, with Japanese teenager Kananko Watanabe shading American Jessica Hardy as quickest in the women's equivalent.
Japanese pair Kosuke Hagino and Daiya Seto topped the men's 400m individual medley heats, while Americans Elizabeth Beisel and Maya Dirado were fastest into the women's 400m medley final.