Starting 9 March 2016, Lee played a five-game match, broadcast live, against the computer program
AlphaGo, developed by a London-based
artificial intelligence firm
Google DeepMind, for a $1 million match prize. He said “I have heard that Google DeepMind’s AI is surprisingly strong and getting stronger, but I am confident that I can win at least this time”. In an interview with
Sohn Suk-hee of
JTBC Newsroom on 22 February 2016, he showed confidence in his chances again, while saying that even beating AlphaGo by 4–1 may allow the Google DeepMind team to claim its
de facto victory and the defeat of him, or even humanity. In this interview he pointed out
the time rule in this match, which seems well-balanced so that both he and the AI would fairly undergo time pressure. In another interview at Yonhap News, Lee Se-dol said that he was confident of beating AlphaGo by a score of 5–0, at least 4–1 and accepted the challenge in only five minutes. He also said "Of course, there would have been many updates in the last four or five months, but that isn’t enough time to challenge me". On 9 March, Lee played black and lost the first game by resignation. On 10 March, he played white and lost the second game by resignation. On 12 March, he played black and lost the third game as well. On 13 March, he played white and won the fourth game, following an unexpected move at White 78 described as "a brilliant
tesuji", and by
Gu Li 9 dan as a "
divine move" and completely unforeseen by him.
GoGameGuru commented that this game was
"a masterpiece for Lee Sedol and will almost certainly become a famous game in the history of Go". Lee commented after the victory that he considered AlphaGo was strongest when playing white (second). For this reason, and because he thought winning a second time with black would be more valuable than winning with white, he requested that he play black in the final fifth game, which is considered more risky when following Chinese Go rules. On 15 March, he played black and lost the fifth game, to lose the Go series 1–4. After his fourth-game victory, Lee was overjoyed: "I don't think I've ever felt so good after winning just one game. I remember when I said I will win all or lose just one game in the beginning. If this had really happened ― I won 3 rounds and lost this round ― it would have had a great bearing on my reputation. However, since I won after losing 3 games in a row, I am so happy. I will never exchange this win for anything in the world." He added: "I, Lee Se-dol, lost, but mankind did not." ==Retirement from professional play==