GWANGJU, South Korea, July 23 (Xinhua) -- China's Yan Zibei qualified for the semifinals with a new Asian record in the men's 50m breaststroke heats at the 18th FINA World Championships here on Tuesday.

Yan closed 26.93 seconds in the preliminaries, ending the race in sixth place. The 23-year-old just won a bronze medal in the men's 100m breaststroke final on Monday, cracking the Asian record set by himself in the semifinals in Gwangju.

"I feel very good after the race. When I swam in the 100m breaststroke heats [on Sunday], I was a bit nervous, but my state of mind got increasingly better," Yan said.

"Yesterday, I fulfilled my wish, so what is left to me is just enjoying the game...This year's championship is a chance to build up my experience for [next year's] Olympic Games...I can gradually improve myself every race," Yan noted.

Despite his record-breaking performance, Yan was not fast enough to overcome Adam Peaty of Britain who finished first in the 50m breaststroke trials with a time of 26.28 seconds.

China's Olympic champion Sun Yang, who won his fourth consecutive world title in the 400m freestyle, hunted for another medal in the men's 800m freestyle, advancing into the final scheduled for Wednesday.

Sun punched the last ticket to the 800m free final with a time of seven minutes and 48.12 seconds. He will compete in the 200m free final later in the day.

"I tried to swim as comfortable as possible in 800m heat because I slept at 2 a.m., this morning and got up at 7:40 a.m. It was my fifth time to swim while most of the others were having their first race. They were in better shape and more energy than me," Sun said after 800m heat.

"My 200m free final is the first event tonight. I don't want to give up any event I compete, no matter 200m or 800m, I tried to make the balance in between the distance," the 27-year-old continued.

In the women's races, U.S. five-time Olympic champion Katie Ledecky withdrew from the 200m freestyle heats and the 1,500m freestyle final for an unspecified ailment.

"A decision has been made by Team USA in consultation with Katie, her coach and the team's medical staff for her to withdraw from the 200m freestyle event on medical grounds," the organization's statement said about one and a half hours before the start of the 200m preliminaries in the morning.

In the women's 200m freestyle trials, Emma McKeon of Australia and Taylor Ruck of Canada also pulled of the races. McKeon and Ruck were among the biggest podium placers in the event.

Ledecky allegedly decided later to scratch in the 1,500m freestyle final in the evening.

The U.S. swimmer has not felt well since she arrived in Gwangju on July 17, according to the organization's statement.

The pullout decision was made two days after Ledecky failed to capture her fourth straight world title in the 400m freestyle. The 14-time world champion was defeated by Australian teenager Ariarne Titmus.

In the men's 200m butterfly preliminaries, Kristof Milak of Hungary sealed first in 1:54.19, followed by Daiya Seto of Japan in 1:54.56.

China's Li Zhuhao and Wang Zhou got knocked out of the heats, finishing in 37th and 40th place each.