Olympic champion Mack Horton has shared a tense reunion with arch nemesis Sun Yang after conceding the 400m freestyle gold to the Chinese star at the world swimming championships in Budapest.

Horton, the Olympic champion, insisted his slow time hurt more than his loss to Sun in the first final of the eight-day titles.

The Australian claimed silver – 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 dismissed Sun as a “drug cheat” at the Rio Olympics before upsetting the defending champion to claim 400m gold.

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

The stage was set for fireworks in the 400m freestyle final after Horton took another swipe at Sun upon his arrival at the world titles.

Asked about Sun, 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.”

However, Sun dominated the 400m medal race to finish more than two body lengths ahead of Horton and claim his eighth overall world titles gold.

Sun let loose a primal scream after his win and sent a glare over to Horton’s lane before raising his finger to remind the Aussie who was the new No.1.

Horton and Sun’s reunion was decidedly awkward. Photo: Getty

The pair later exchanged an awkward handshake on the medal podium.

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

“I think the time stings a bit more,” he said.

“I thought I was capable of more tonight.”

Horton, 21, also wouldn’t bite when asked about Sun working with Australian coach Denis Cotterell.

“You are touching on my sore spots,” he said.

Just as during the Rio Olympics, Sun supporters were vocal and ruthless in their social media attack on Horton.

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Australia had to wait until the final event of the opening night to claim another silver when the depleted women’s 4x100m freestyle relay team finished behind a Katie Ledecky-led United States.

The Aussie women’s sprint relay team of Shayna Jack, world champion Bronte Campbell, Brittany Elmslie and Emma McKeon dug deep without their leader Cate Campbell but just fell short of the US.

The Aussie women had won every major 4x100m freestyle relay event since 2014 but could not reel in the US without ex-world champ Cate Campbell who has taken a 2017 sabbatical.

Cate Campbell’s 100m freestyle world record was earlier shattered in the relay final by Sweden’s lead-off swimmer Sarah Sjostrom (51.71), who became the first woman to break the 52 second barrier.

Campbell had set the previous mark of 52.06 before Rio.

The new-look Australian men’s 4x100m freestyle relay team without Olympic champion Kyle Chalmers (heart) touched the wall fifth in their final but were disqualified for a false start.

The 16-year-old Ariarne Titmus – one of 13 rookies on the Dolphins team – was fourth in the women’s 400m freestyle final behind Ledecky.

And McKeon – Australia’s most decorated Rio swimmer with four medals – qualified second fastest for Monday night’s 100m butterfly final by equalling Jessicah Schipper’s 2009 national record (56.23).

– With AAP