It left Jack, 16, a Form Five student at CCC Kei Heep Secondary School, in Wong Tai Sin, struggling for confidence. But he was determined to bounce back. And sure enough, 12 months later, he claimed that elusive gold medal.
Jack had expected the 2010 race on a course around Aberdeen Reservoir to be no problem for him.
"I was on top form and had achieved many personal-best times in different track events," he says. "So I was feeling really confident and thought those up against me were not going to be too testing ... I thought I would win the title easily."
Over-confidence may well have got the better of him and led him to collapse exhausted at the finish.
Jack's coach, Alexander Chau Chun-ho, had worked out a detailed race strategy for him. But Jack ignored the plan from the start. "I didn't follow his instructions, I'm afraid," Jack says. "Instead of pacing myself, I kept on accelerating - just what I was not supposed to do."
The Y-shaped course starts and finishes at the same place: competitors run to the tip of the left-hand fork of the Y, then back to the start before racing to the point of the right-hand fork, then sprinting to the finish, at the base of the Y.
Jack's reckless sprinting meant he was tired after he had run up the steep slope of the right-hand fork and began sprinting to the finish.
"I wanted to give up at one point because I was so tired, but I told myself to keep going as not many runners would catch me," Jack says. "I saw one runner go past me into the lead as I approached the line, but have no recollection of a second competitor going past. My mind went blank and I fell unconscious."
A first-aid team gave him oxygen before he went to hospital for a check-up. Medical staff were so concerned that they kept him in overnight for observation at Queen Mary Hospital, in Pok Fu Lam.
In the hospital, Jack admitted to his coach he had ignored his plans, but Chau did not blame him. "He was very encouraging," Jack says. "He just told me to get well."
However, back at school, some of Jack's classmates made fun of him over what had happened. "They warned me to be careful even when we jogging during the warm-up in PE lessons. They were only teasing, but I felt embarrassed about what had happened."
Although Jack is his school's best middle- and long-distance runner, he struggled to regain his confidence. "My running was off for the rest of 2010 and right up until last summer," he says. "I kept on training, but I didn't feel motivated to race."
Chau, who has trained Jack for the past three years, is full of praise for his attitude. "Jack is mentally tough and very mature as a runner," he says. "Although he hadn't done as well as he had hoped - and had lost confidence - to his great credit he kept on training."
His perseverance paid off when, 12 months on, a determined Jack lined up for the same 6km race last October, having been promoted to the Grade A competition for older runners born in 1995 or before.
"The race was over the same course," Jack says. "I arrived early, just like I have done at the same competition in previous years. I was nervous and really worried that I might I make a mess of things - as I did in 2010."
However, Jack had learned his lesson - and paced himself from the start alongside two other runners. He began to sprint only as he reached the final straight heading for the finish line. "I took off at full speed and it felt exciting knowing that I had got my confidence back, and finally, I had overcome the disappointment of what happened."
Jack says he was surprised how well he felt at the finish - unfazed by his exertions - as he claimed the winner's gold medal.
Since his victory, he has focused on new challenges. "I want to get closer to the times of leading junior runners in local events," he says. "After learning my lesson, I'm more confident about fulfilling my many dreams and goals."