Top-ranked Lee, twice a runner-up at the tournament, brushed aside young Danish hopeful Viktor Axelsen 21-9, 21-7 in front of a partisan Copenhagen crowd to move within touching distance of a first major title.
Despite his advancing years Lee, who will turn 32 in October, demonstrated his enduring class with a sublime blend of blistering overheads and resolute defence to reel off 12 of the final 13 points in the first game and never looked back.
"So far things have got better each match," said Lee, who will face the winner of the match between Chinese second seed Chen Long and Indonesia's fifth-ranked Tommy Sugiarto in Sunday's final.
"I can go back to resting now and prepare for tomorrow.
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"The country will be watching and there will be more pressure on my shoulders. I will try not to think about the pressure and give a big present to my country," added Lee, who is hoping to give people back home further cause for celebration on Merdeka Day, which marks Malaysia's 1957 independence from British colonial rule.
World number one Li, runner-up a year ago on home soil in Guangzhou, dominated from the beginning with Mitani, sporting a heavily strapped right leg, unable to offer much resistance as Li powered to a 21-8, 21-14 victory.
Mitani had claimed the scalp of defending champion Ratchanok Intanon of Thailand in the last 16 before upsetting South Korean fifth seed Sung Ji-hyun in the quarters but there were no repeat heroics this time.