The disgraced kayaker caused his rival to fail a doping test, after lacing his drink with an anabolic steroid, the Japan Anti-Doping Agency said on Tuesday.
Though Komatsu was suspended after failing the drug test, his repeated denials of ever taking steroids ultimately prompted officials from the Japan Canoe Federation (JCF) to launch an investigation into the matter, Japanese outlet NHK reported.
Japanese kayaker, Yasuhiro Suzuki, has been banned from competition for eight years for drugging a fellow competitor.
In the 2017 Canoe Sprint Japan Championships, held in Ishikawa Prefecture in September, both Suzuki and Komatsu took part in the kayak single 200 meters event.
At the time, the two sportsmen, who were teammates in last year's world championships, were widely regarded as Japan's top prospects to represent the country at the 2020 Tokyo Olympics.
"I was fretting", Suzuki was quoted as telling the federation.
When confronted about the allegation, Suzuki admitted to spiking Komatsu's drink after he'd "felt guilty".
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The Japan Canoe Federation also found that Suzuki, 32, had sabotaged other athletes' chances at multiple competitions by stealing others' tools or gear, NPR reported.
A TOP Japanese canoeist has been banned for EIGHT years after he spiked a rival's drink in what has been labelled an "extremely evil" act.
Suzuki has admitted drugging his rival.
Murofushi said when he competed as an Olympic hammer thrower, he had someone watch his meals at athletes' village when he had to leave the table.
He has previously stolen opponents' equipment and now caused Japan's first instance of an athlete failing a doping test due to deliberate contamination.
Komatsu himself told the media on Wednesday, "I did not think this could happen in Japan". "If Suzuki had not confessed, I would have faced a long suspension so I guess on that point I have to thank Suzuki for coming clean and confessing".