Stricken by fever, an under-the-weather Joseph Schooling could only manage a lukewarm performance in his first major meet of what could be a pivotal year for himself and Singapore sports.
The swimmer clocked 54.10sec in the 100m butterfly heats at the Arena Pro Swim Series in Austin, Texas on Friday morning.
His timing was the 14th-fastest at the Lee and Joe Jamail Texas Swimming Centre and meant he missed out on Friday night's A final, qualifying only for the B final.
Schooling, who holds the Asian record of 50.96, which he set en route to a bronze medal at last year's World Championships, subsequently withdrew from the race.
The University of Texas at Austin student, 20, had come down with a virus since last week and had a 38.9 deg C fever and only returned to light training on Monday.
He told The Sunday Times: "In this morning's heats, I tried to push myself but I know I was not 100 per cent. Although disappointed with the time, I came into the race just wanting to have a feel of the water and see how far I can go.
"I spoke to (Texas head coach) Eddie (Reese) and he advised me to rest and not swim in the final."
Compatriot Quah Zheng Wen (53.33) finished eighth in the A final which featured a world-class field. American superstar Michael Phelps won in 51.94, ahead of United States team-mates Tom Shields (52.39) and Tim Phillips (52.61).
Said Quah, 19: "I wasn't too happy with my timing but it was good to get some racing experience against these guys.
"I'm just coming out of a training camp with the national team and so I wouldn't read too much into my timings at the moment."
In the 100m freestyle event, Quah clocked 50.51 and finished fourth in the C final. He competed in the 200m free and 200m backstroke events early this morning.
Team-mates Lionel Khoo and Pang Sheng Jun were fifth in the 200m breaststroke C final (2min 18.77sec) and sixth in the 400m free D final (3:59.10) respectively.
Both Quah (100m back, 100m & 200m fly) and Schooling (100m free, 100m & 200m fly) have already qualified for three events each at the Olympic Games in Rio in August.
Despite the modest timing in his pet event, Schooling, who carries Singapore's hopes of a historic Olympic swimming medal, remained undeterred.
Originally slated to compete in both the 100m and 200m fly events in Austin, he decided to skip the latter. The three-day meet ends today.
He said: "For me what is important is I stick to my game plan. The less I worry about others, the better it is for me.
"It is always great to swim in a field of world-class swimmers... But me and Eddie we have worked on a plan up to the Olympics.
"I won't consider this as a setback but more towards a gauge of where I am with the rest of the flyers.
"The NCAAs in March will be one of the milestones for me this year and my focus is on that for now."
The Republic's national coach Sergio Lopez also believes that once fully fit, Schooling would regain the form he showed at the World Championships in Kazan, Russia.
"Joseph has been sick for more than a week. It was a hard virus and he lost a bit of weight as a result.
"He's still trying to get his feel of the water and racing back again but physically, he looks very strong."