RIO DE JANEIRO -- Adam Peaty of Britain set a world record of 57.55 seconds in the men's 100m breaststroke heats on Saturday, the first day of the swimming competition of the 2016 Rio Olympics.

The triple gold medalist of the 2015 World Championships didn't know how he did it as well.

"I don't know. I mean I went out pretty fast, quite easy, came back and I heard everyone cheering. I thought what they are cheering for, as there's no Brazilians in this lane?" said the 21-year-old swimmer.

In other heats of the day, China's Sun Yang made the men's 400m freestyle final, but his compatriot Ye Shiwen didn't make the women's 400m individual medley final.

Defending champion Sun clocked 3:44:23 to advance to men's 400m freestyle final, which will take place in the evening, while Corner Dwyer from the United States finished first, 0.81 seconds ahead of Sun.

Park Tae-hwan of South Korea, the Olympic champion in Beijing of this event, didn't make the final, clocking 3:45:63 while competing in the same heat with Sun.

China's 20-year-old Ye, the defending champion of women's 200m and 400s individual medley, finished seventh in the 4th heat of women's 400m individual medley, failed to make the final. Zhou Min also failed to make the final in the same event as well.

China's Wang Shun finished 10 in the men's 400m individual medley, and Qiu Ziao clocked 3:49:45 in men's 400m freestyle, and was stopped before the final round as well.

However, two Chinese girls Lu Ying and Chen Xinyi qualified for the semifinal of women's 100m butterfly, clocking 57:98 and 57:17 respectively.