Sun, who has already won gold in the 400m and 800m, had no problems qualifying for Sunday’s final, despite swimming well below his best.
Sun posted a time of 14 minutes 55.11 seconds, more than 23 seconds outside the world record he set at the 2012 London Olympics,
Sun won the 1,500m at the last two world championships but was conserving as much energy as he could on Saturday in anticipation of a tough final against Italy’s Gregorio Paltrinieri and American Conor Jaeger, who qualified ahead of him.
The morning after Australia’s Campbell sisters, Bronte and Cate, raced to gold and bronze respectively in the women’s 100m freestyle final, the pair were back in action in the 50m freestyle heats.
Racing in adjoining lanes, Cate took top honours, posting 24.40 seconds to qualify fastest for the semi-finals despite a terrible start off the blocks.
Her younger sibling qualified seventh quickest in 24.74 seconds.
Arianna Vanderpool-Wallace of the Bahamas touched 0.03 seconds behind cate, with Sarah Sjostrom qualifying third in 24.53 seconds.
“I’m glad to be through as it would have been a screw up if I hadn’t,” said Bronte Campbell, who succeeded her sister as world champion in the blue riband 100m event.
Ruta Meilutyte of Lithuania set down a marker in the women’s 50m breaststroke, topping the time sheets with a 29.74, the only swimmer to break 30 seconds.
Meilutyte lost her 100m breaststroke title on Tuesday to Russia’s Yuliya Efimova, who was returning to competition five months after completing a 16-month ban for taking steroids.
“Emotions have calmed down a little bit,” Meilutyte told reporters. “Everybody seems to be more relaxed which is good. At the beginning of the championship it was tense for me in the 100m.”
After failing to qualify for the women’s 200m breaststroke final, defending champion Efimova said,” I am upset that I have lost my chance, but I think that I can beat the world record in the 50m.”
In the men’s 50m backstroke, Camille Lacourt of France touched in 24.56 seconds to secure a semi-final berth, recording the fastest time in qualifying. American Matt Grevers was 0.12 seconds behind.
Ryan Lochte helped the United States post the fastest qualification time in the mixed 4x100m freestyle relay ahead of the final later on Saturday. The U.S posted three minutes 24.51, Russia and Italy qualifying second and third respectively.
Lochte’s compatriot Katie Ledecky will later on Saturday bid to become the first swimmer in both genders to win all the freestyle events from 200m through to 1500m when she competes in the women’s 800m final.
(Editing by Amlan Chakraborty and Julian Linden)