SINGAPORE - National marathoner Mok Ying Ren's bid to qualify for the Rio Olympics suffered a setback on Sunday after a disappointing performance at the Lake Biwa Marathon in Otsu.

The 27-year-old, whose personal best is 2hr 26min 30sec from the Gold Coast Marathon in 2013, clocked 2:35 for the IAAF Gold Label race in the capital of Japan's Shiga Prefecture.

The qualifying mark for the Aug 5-21 Summer Games is 2:19 with the closing date on July 11.

Mok had taken one year off from his residency in orthopaedic surgery with the National University Health System to train with the Boulder Track Club in Boulder, Colorado. He works under three-time Australian Olympian Lee Troop.

Mok, the 2013 SEA Games marathon champion, wrote on his Facebook page: "Utterly disappointed with my performance today at the Lake Biwa Marathon. I'm sorry to disappoint all my close friends, supporters including @sporeathletics and sponsors @airasiasg @newbalancesg @oakleysg @pocarisweatsingapore @travelmadedifferent @fitnessfirstsg @brands_sg.

"The final 10km was the toughest I had ever run to complete with a time of about 2hr 35min. Wanted badly to stop by the road side but I knew I had to finish what I set out to do.

"Training had been going well in Boulder under @leetroop73 and I cannot explain this poor performance. No excuses and to focus on recovery now.

"Now, time to enjoy Kyoto with my fiancée @speedbel for putting up with my training overseas for the past year. And then back to Boulder again to continue working on it."

Kenya's Lucas Rotich won the Lake Biwa event in 2:09.11, edging Japan's Hisanori Kitajima by five seconds in a tight finish. Alphonce Felix Simbu of Tanzania was third, a further eight seconds back.