RIO DE JANEIRO - National swimmer Quah Zheng Wen finished a disappointing 22nd in the heats of the men's 100m backstroke event on Sunday at the Rio Olympics.

The 20-year-old, starting in Lane 8, clocked 54.38sec in Heat 4, and finished seventh of eight. American David Plummer won the heat in 53.19s.

There were 39 participants and only the top 16 advance to the semi-finals at the Olympic Aquatics Stadium in Rio.

French swimmer Camille Lacourt was the fastest with a 52.96s effort while New Zealand's Corey Main was the last semi-finalist with a time of 53.99s.

Quah's time was 0.35s slower than his personal best and national record of 54.03s. He had the slowest reaction time in his heats and clocked a slow 26.34s split.

He did not speak to the media after his event.

National coach Sergio Lopez was surprised by the result and had expected Quah to post a sub-53s timing and rewrite his national mark.

The Spaniard said: "He trained very good, did a good job so far. I don't know what happened. He said he felt he didn't have his speed, felt a bit slow coming out, his legs were a bit tight...

"He seemed calm, composed and I thought he was going to do better."

Quah, who won 12 medals at last year's SEA Games and seven medals during last season's Fina World Cup series, has two other events in Brazil, the 100m and 200m butterfly.

He will next compete in the 200m fly on Monday afternoon (Rio time) and Lopez expects a better showing.

He said: "His 200m fly is what we trained for. We trained for everything of course but didn't do anything specific for 100m back. The best chance he has to do something is the 200m fly."

He added that Quah's confidence would not be shaken by this result. He said: "We have time, he'll warm down and let it out. He'll come back. If he doesn't bounce back with confidence, he's going to miss out on a chance to do something.

"Once you get here, it's not so much about the training, it's about how big your cojones are..."

Quah's sister Ting Wen had competed on Saturday in the 100m fly and clocked 1min0.88sec, more than a second slower than her qualifying time for Rio, 59.83s. She was third in Heat 2 and did not progress.

On helping his swimmers deal with disappointment, Lopez said: "As a coach, you try to make them understand, they didn't fail. It's part of life. You go, you give your best, you keep trying."

The third member of Singapore's three-man swim contingent, Joseph Schooling, opens his Olympic campaign in the 100m freestyle event on Tuesday afternoon. He will also swim in his pet event, the 100m fly on Thursday.

Follow the latest Olympics 2016 news here at str.sg/olympics.