The human Go champion said he was left "speechless" after his second straight loss to Google's Go-playing machine on Thursday in a highly-anticipated human versus machine face-off.
Lee Sedol, one of the best Go players in the world with 18 world championships, was grim and ashen after the game, which lasted more than four hours.
"I'm quite speechless," he said at the press conference that was livestreamed on YouTube. "It was a clear loss on my part. From the beginning there was no moment I thought I was leading."
The latest victory by AlphaGo over South Korea's Lee puts Google's AlphaGo team one victory away from claiming the $1 million prize.
If AlphaGo wins, the prize money is to be donated to UNICEF, Go organizations and charities.
AlphaGo's first win against Lee, on Wednesday, shook the Go-playing world, marking a milestone in the development of artificial intelligence.
Hundreds of thousands of South Koreans, even those who do not play the popular board game, followed the games on live TV and on YouTube. All major local newspapers reported Lee's loss on their front pages.
"2-year-old artificial intelligence masters 5,000-year-old human Go," said a headline in the Chosun Ilbo, South Korea's most circulated daily newspaper.
Many had believed it would take another decade for computers to conquer the ancient Chinese board game, one of the most creative games ever devised and the only board game left to conquer after chess was mastered by computers in 1997. Go is much more complex than chess.
Google's team compared AlphaGo's win to landing on the moon.