It has already been dubbed "the War on Water II".

But Olympic champion Mack Horton still managed to pump up his rivalry with Sun Yang even further ahead of the world swimming championships in Budapest, by again dismissing him as a drug cheat.

The stage was already set for an explosive reunion in Hungary between Australian Horton and seven-time world champion Sun after they famously locked horns at the Rio Olympics.

Horton caused a sensation at the 2016 Games when he slammed Sun, saying he had "no time or respect for drug cheats" before upsetting the Chinese champion in the 400m freestyle gold medal race.

And Horton could not help but have another dig before kicking off the eight day titles on Sunday (AEST) in the 400m freestyle when he lines up against defending champion Sun for the first time since their Rio showdown.

Asked if he looked forward to resuming his rivalry with Sun in Hungary, 21-year-old Horton said: "I don't know if it is a rivalry.

"I think it is a rivalry between clean athletes and athletes who have tested positive but there should be some good racing."

Flying the flag for clean sport has come at a price for Horton.

Mack Horton could not help having a dig at Sun ahead of their first post-Rio showdown. ( AAP: Glenn Hunt )

China's swimming team demanded an official apology at the Rio Games over his comments while Horton is still bombarded by vile abuse from online trolls.

Asked if he had any regrets over his "drug cheat" call after enduring the fallout, Horton said: "I think I would do the same thing even if I knew the outcome.

"I don't think that (reaction) changes anything."

Horton called his 400m gold medal victory in Rio over Sun a "win for the good guys", referring to the Chinese champion serving a three-month ban for testing positive to a banned stimulant in 2014.

Three-time Olympic gold medallist Sun raised eyebrows when he served a three month ban in 2014 in secret and returned to competition before his suspension was officially confirmed by Chinese officials.

Horton's remarks about Sun (pictured) caused an uproar in China. ( Reuters: Toby Melville )

Now a member of the Australian team leadership group, Horton is set to clash with Sun in the 200m, 800m and 1,500m freestyle events in Budapest.

The confident Horton is very different to the Australian rookie who made his world titles debut in 2015 in Kazan.

Horton failed to make the 400m and 1,500m finals in Russia after being struck down by an infection but somehow managed to bounce back with 800m freestyle bronze.

"When I think about that 2015 experience, it was more that's what losing feels like," Horton said.

"I know that if I do lose it's not the end of the world, so don't stress, relax, it's going to be okay.

"That's what I took away from 2015."

AAP