Ryan Lochte wrote himself into the history books on Thursday while Katie Ledecky won her fourth gold at the world swimming championships to cap a great night for the United States.

Lochte's victory in the 200m individual medley final saw him become the first swimmer to win four consecutive world championships gold in the event.

"It's something I'll remember for the rest of my life," said Lochte.

He now has 16 gold medals at world championships dating back to 2005.

Australia's Grant Hackett is the only other swimmer to have won four consecutive world titles in a single event, having achieved the feat in the 1,500m freestyle between 1998 and 2005.

"It would have to be a toss up between this, it's the coolest thing to make history in any sport, or 2011, when I broke the (200m IM) world record," said Lochte when asked if this was the best moment of his illustrious career.

- Phelps' SMS -

Lochte revealed US swimstar Michael Phelps, who was sanctioned from taking part in Kazan, told him to take leadership of their country's team via a pre-race text message.

The 31-year-old Lochte produced a commanding display to win his final and both butterfly expert Camille Adams and breaststroker Micah Lawrence said Lochte's victory had inspired them to finish second in their following races.

Ledecky then anchored the United States women's 4x200m freestyle relay victory.

The Americans finished the night with two golds and Adams' silver in the women's 200m butterfly to move up to second in the overall medals table behind China.

The 18-year-old Ledecky had already won the 200, 400 and 1,500m freestyle golds in Kazan and still has the 800m heats to come on Friday morning and final scheduled for Saturday night.

After Missy Franklin swam a strong first leg in the relay, Ledecky dived in with the United States slightly behind the Swedish team, but she put plenty of clear water between her team's rivals to win by a clear margin.

Ledecky, who holds the world records for the 400, 800 and 1,500m freestyle, has had an outstanding campaign in Kazan and has twice broken her own 1,500m record.

"There's only one word I can use to describe Katie Ledecky - 'phenomenal'," said Lochte.

"It seems whenever she sets foot in the water she either wins gold or breaks a world record -- she's a beast.

"She got Team USA on a roll and we have to carry on that momentum."

It was also a good night for Asia as Ning Zetao became China's first 100m freestyle world champion, while Fu Yuanhui won women's 50m backstroke gold with fellow 19-year-old Liu Xiang earning bronze for the Chinese team.

Japan's Natsumi Hoshi won the women's 200m butterfly gold to add to her 2012 Olympic bronze in the event while 17-year-old Chinese Zhang Yufei swam a new world junior record of 2:06.51 to take bronze.

- freestyle battle royal -

The women's 100m freestyle final on Friday will be a huge battle with Australian sisters Cate and Bronte Campbell, plus Holland's Femke Heemskerk, all within 0.60 secs of Sarah Sjostrom's 52.78secs fastest time in the semi-finals.

Cate Campbell is the defending champion.

Denmark's world record-holder Rikke Moller Pedersen is on course to win women's 200m breaststroke gold after qualifying fastest from the semi-finals just ahead of Lawrence.

Pedersen, who set the world record in Barcelona two years ago, is now the favourite after Russia defending champion Yuliya Efimova failed to qualify.

Mitchell Larkin is on course to add 200m backstroke gold to the 100m title he has already won after qualifying as the fastest into Friday's final.

He was just ahead of the United States' Ryan Murphy and China's Xu Jiayu. Olympic champion Tyler Clary of the USA was seventh fastest.(+++)

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