KAZAN, Russia - China's Sun Yang was only third in Saturday's 1500m freestyle heats as the long-distance expert bids to complete the treble of titles at the world swimming championship.
The 23-year-old Sun has already won the 400 and 800m freestyle golds in Kazan, plus a silver in the 200m, and is looking to again complete the long-distance treble of world titles he won in Barcelona two years ago.
But he faces a fight over the mile in Sunday's 1500m final as Italy's Gregorio Paltrinieri, who swam the world leading time coming into Kazan, clocked the fastest time of 14 minutes, 51.04secs in the heats.
Connor Jaeger of the USA was second fastest at 2.30secs with Sun third at 4.07 and the Chinese giant has already identified Paltrinieri as his biggest threat.
Sun's fellow long-distance expert, Katie Ledecky of the USA, is poised to make history in the women's 800m final on Saturday evening.
The 18-year-old is bidding to sweep the freestyle medals having already secured the 200, 400 and 1500m world titles in Kazan.
No swimmer has ever achieved the golden sweep of four individual freestyle titles at a world championships.
Fresh from taking bronze in the women's 100m freestyle final on Friday, Australia' Cate Campbell was fastest into Saturday night's 50m semi-finals.
Campbell clocked 24.40 seconds to lead a strong field, while younger sister Bronte, who succeeded her sibling Cate as the new world 100m champion, was seventh fastest at just 0.34 back.
Campbells storm into semis
"I didn't get to bed until 0230 this morning, so it was good just to get through the rounds, it would have been a really big screw up if I hadn't," said Bronte with the 50m final on Sunday.
"Last night was pretty amazing, but I can't think about it too much.
"I haven't really looked at my phone, because it'll be a distraction if I start reading the messages, there are a lot on it, and my job is only half done.
"I still have two 50m races to go and probably a job to do in the relay." In the men's 50m backstroke heats, Australia's double champion Mitchell Larkin was fourth fastest in his bid to add the sprint gold in Sunday's final to the 100 and 200m world crowns he has already won in Kazan.
Reigning world champion Camille Lacourt of France was the fastest through in 24.56 secs with Larkin at 0.21, despite the exertions of winning Friday night's 200m and Tuesday's 100m finals.
"To get the 100 and 200m golds was amazing, the 50 is a whole different kettle of fish, but anything's possible," said the 22-year-old Larkin.
"I felt good this morning and slept well, surprisingly. I tried to forget about winning last night and just get ready for today." World record-holder Ruta Meilutyte, 18, was the fastest into the women's 50m breaststroke semi-finals as she blasted out in 29.74 secs - just 0.26 away from her own record.
Russia's Yuliya Efimova, who snatched Meilutyte's world 100m crown in Tuesday's final, was fifth fastest at 0.71secs in her first major meet since serving a 16-month suspension for taking a banned steroid.
Efimova said failing to qualify from the 200m breaststroke heats on Thursday has given her extra motivation to attack Meilutyte's 50m world record of 29.48 secs, set in Barcelona two years ago.
Efimova eyes record
"I was upset to miss out in the 200m and I know I am capable of swimming faster," said the Russian.
"It upset me and I think I can beat the 50m world record." The pair will be joined in the battle for the breaststroke sprint title in Sunday's final by Jamaica's Alia Atkinson, who took bronze in the 100m final and was 0.53 back in the 50m heats.
The 2011 world champion Jessica Hardy of the USA was 0.70 back.
There were no surprises in the mixed 4x100m freestyle relay heats as the USA team, which included superstar Ryan Lochte, swam the fastest time of 3:24.51 ahead of Russia, Italy and Sweden.