Australian assistant Michael Bohl claimed Rio 400m gold medallist Horton had not breathed a word about his arch-rival being cleared to contest the eight-day world titles trials in Gwangju, South Korea starting on Sunday.

It seems Horton is in the minority.

Sun has created dismay from critics ahead of the world titles after being given the green light to compete following a delay in setting a date for his latest doping case.

Sun - who served a secret three month doping ban in 2014 - faces a lifetime suspension after allegations a vial of his blood was smashed by a hammer during a clash with testers in China last year.

Reports claimed Sun objected to an out-of-competition test at his Zhejiang home last September and it was alleged his mother ordered security guards to destroy a vial of the swimmer's blood that had been taken.

World body FINA decided not to reprimand Sun amid claims the testers had not shown adequate identification but World Anti-Doping Agency lodged an appeal.

However, Sun will still compete at the world titles with his Court of Arbitration for Sport tribunal date yet to be finalised.

It seemed it would be enough to earn the ire of Horton, who called Sun out as a "drug cheat" before upstaging the Chinese great to claim Rio 400m gold.

But it seems Horton is one of the few to register Sun's latest controversy as the Australian tries to regain his mojo at Gwangju.

"I am actually looking after Mack here. I haven't spoken a word to Mack about it (Sun) and he hasn't spoken a word (about Sun) - he (Horton) is a real cool customer," he said.

"He stood up for what he believes in at the last Olympics and said what he thought and that is the end of it.

"But Mack is here to do the best job that he can because he didn't have a great trials."

Indeed Horton clearly has more immediate problems to tackle than Sun.

Remarkably Horton only got on the world titles team after head coach Jacco Verhaeren used his discretionary powers to grant the 23-year-old a spot following a shock trials display.

Horton failed to qualify in the 200m, 400m and 800m but still had his name read out with the Dolphins team.

Verhaeren has backed Horton by entering him in the 400m and 800m freestyle events at South Korea.

Bohl expected Horton to lift at Gwangju but admitted coaching staff still had no clue as to why his star charge suffered his trials meltdown.

"I have no idea. But he just didn't have a good meet. But you can't accidentally win a gold medal," Bohl said of Horton's pedigree.

Australian Associated Press