Sun Yang of China reacts after the men's 400m freestyle final at the Gwangju 2019 FINA World Championships in Gwangju, South Korea, July 21, 2019. (Xinhua/Li Gang)

GWANGJU, South Korea, July 21 (Xinhua) -- China's Olympic champion Sun Yang won his fourth straight world title in the men's 400m freestyle on the first day of the swimming events at the 18th FINA World Aquatics Championships here on Sunday.

The 27-year-old Chinese swimmer finished first in the 400m free final, clocking 3:42.44. It was his fourth consecutive gold medal in the event at the FINA worlds since 2013.

Mack Horton of Australia touched home as runner-up in 3:43.17, and Italy's Gabriele Detti finished in 3:43.23 for the bronze. China's Ji Xinjie ended the 400m free final in seventh place with 3:45.64.

Supported by a large contingent of Chinese fans, Sun met expectations as he won the first gold medal on the opening day of the swimming events at the worlds.

"I wanted to show myself today, step onto the podium and watch China's national flag rising," said Sun. "It is a great honor for me."

"The training was really tiring. Perhaps only athletes know their own efforts behind the medals. As a senior athlete, I feel proud of myself as I have kept fighting until now," said the triple Olympic champion.

U.S. swimming star Katie Ledecky's quest to claim her fourth straight world title in the women's 400m freestyle was thwarted by Australia's rising star Ariarne Titmus.

Titmus sealed first in the final with a time of 3:58.76, 1.21 seconds ahead of Ledecky, who had taken gold medals in the 400m, 800m and 1,500m free at the past three straight world championships from 2013 to 2017.

Leah Smith of the United States took bronze, and China's Wang Jianjiahe finished fifth, failing to reach the podium.

In the men's 4x100m freestyle relay, the U.S. team took gold in 3:09:06, followed by Russia in 3:09.97 and Australia in 3:11.22 to secure silver and bronze medals.

Australia's freestyle relay team snatched gold in the women's 4x100m race with a time of 3:30.21, less than a second faster than the U.S. team who took silver. Canada came away with bronze in 3:31.78.

Katinka Hosszu of Hungary placed first in the women's 200m individual medley (IM) semifinals, followed by Sydney Pickrem of Canada and Melanie Margalis of the United States.

China's Ye Shiwen ended the women's 200m IM semifinals in fourth place, qualifying for Monday's finals.

Caeleb Dressel of the United States, who secured seven titles in Budapest two years ago, took the lead in the men's 50m butterfly semifinals, easing into Monday's final in a time of 22.57.

Nicholas Santos of Brazil and Andrii Govorov of Ukraine followed in 22.77 and 22.80 respectively.

In the women's 100m butterfly, Sarah Sjoestroem of Sweden ended the semifinals in first place, followed by Margaret Macneil of Canada and Marie Wattel of France.

Adam Peaty of Britain broke a world record, ending the men's 100m breaststroke semifinals in 56.88. China's Yan Zibei followed with a time of 58.67, followed by James Wilby of Britain in 58.83.

For the swimming events, a total of 42 gold medals are up for grabs. Team China sent a squad of 43 swimmers to the world championships.