Sun got the better of Horton in the men's 400m freestyle final to the delight of a large Chinese contingent in the stands, with the Australian cutting a frustrated figure after being pushed into second.

Chinese swimmer Sun was accused of deliberately smashing his blood sample in a row with drugs testers earlier this year but avoided punishment from the International Swimming Federation (FINA) and its doping panel, instead receiving a warning.

The World Anti-Doping Agency (WADA) has since filed an appeal against this decision at the Court of Arbitration for Sport (CAS).

If CAS rules in WADA's favour, Sun could receive a lifetime ban, having already served a three-month suspension in 2014 after testing positive for prohibited substance trimetazidine.

His permitted entry to the World Championships has been heavily criticised by his rivals, with Horton one of the most outspoken.

After the race, Horton said: "I think you know how I feel.

"I think you all know what the rivalry is like.

"His actions, and how it's been handled, speak louder than anything I’ll have to say."

Horton returned to the arena to collect his silver medal but refused to climb onto the podium next to Sun, instead receiving his medal on the ground.

Sun took gold in three minutes 42.44sec, with Horton second in 3:43.17.

Gabriele Detti of Italy completed the podium in 3:43.23.

A dramatic first night in the pool saw United States superstar Katie Ledecky defeated by Australian Ariarne Titmus.

Five-times Olympic champion Ledecky failed to win a world final for just the second time in her career as 18-year-old Titmus stormed past her in the closing stages.

Titmus clocked 3:58.76 to win gold, with Ledecky coming home for silver in 3:59.97.

The bronze medal went to American Leah Smith in 4:01.29. 

"I’m very happy," said Titmus.

"I knew I had to take it out hard to be with her and her third hundred is still better than mine.

"I saw her move and I thought to myself 'hang in there, she’s going to have more and you’re not going to get her', but suddenly I found something and it feels good."

Briton Adam Peaty finally broke through the 57-second mark at Nambu University Municipal Aquatics Center with a 100m breaststroke world record in his semi-final.

He lowered his previous record of 57.10 to touch home in 56.88, ending a three-year wait to duck under 57 seconds.

After his heat this morning, Peaty told insidethegames he was not going to hunt the record anymore, but instead let it come naturally.

Just a few hours later the Briton was beaming and wants to go faster again in tomorrow's final.

"It is incredible," said Peaty.

"I've been chasing it for three years now, ever since I touched the wall in Rio I knew I could go faster. 

"It always comes down to where and when you do it, though.

"I’ll hopefully go faster tomorrow."

In the men's water polo crossover matches, Australia needed a shootout to see off Montenegro to reach the last eight.

The match was tied at 9-9 before the Australians recorded a 13-11 victory.

Greece won their last quarter 3-1 as they defeated the United States 11-9 to progress to the quarter-finals.

Spain won 15-7 against Japan and Germany crushed South Africa 25-5 in sealing their passage to the next round.

Kazakhstan defeated hosts South Korea 17-6 to book their place in Tuesday's classification match for 13th place.

They will take on Brazil after they overcame New Zealand 12-8.