Budapest: Olympic champion Mack Horton insisted his slow time hurt more than arch nemesis Sun Yang denying him 400m freestyle gold at the world swimming championships at Budapest on Sunday night.

On the opening night of the eight-day titles, Horton clocked three minutes, 43.85 seconds to claim the 400m silver medal - almost two and a half seconds behind China's controversial winner Sun at Duna Arena.

The 400m freestyle medal race was billed as "the War on Water II" after Horton's memorable clash with Sun at the Rio Olympics.

Horton caused a sensation on his Olympic debut when he dismissed Sun as a "drug cheat" then upset the defending champion to claim 400m gold.

Sun served a secret three-month suspension for testing positive to a banned stimulant in 2014.

But Horton said he was more frustrated by his slow time - almost two and a half seconds behind his personal best - than Sun defending his 400m world title.

Asked how much it stung to see Sun win, Horton said: "I think the time stings a bit more.

"I thought I was capable of more tonight.

"I was too soft, I need to be harder on myself.

"But this is the start of the cycle, this is the time I have to work on towards Tokyo (Olympics in 2020)."

Horton also wouldn't bite when asked about Sun working with Australian coach Denis Cotterell.

"You are touching on my sore spots," Horton said.

LC/WD