"A tough guy to beat," Li said of three-times major champion Koepka, the third seed.
"It was incredible. I didn't expect that.
"I was in some bad positions today, but I felt I stayed patient, did that very well."
With a perfect 2-0-0 record, 36th seed Li only has to halve with English 60th seed Tom Lewis on Friday to win his group and advance to the round of 16.
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Koepka, with one loss and one halve, will head home on Friday no matter the result of his final match of the round-robin phase against Swede Alex Noren (1-1-0).
Noren can still proceed to the round of 16, but the cards must fall his way.
Li suggested the match play format suited his mindset.
"There's less pressure, actually, at some points," he said.
"Especially when you have a bad hole, you always can play well on the next one and get something back, so you just have to hang in there and do your best.
"I feel nothing to lose ... Lose a hole, play the next one."
(Reporting by Andrew Both in Cary, North Carolina)
