After three straight defeats, Korean Go master Lee Se-dol finally won a victory against Google’s artificial intelligence AlphaGo in their fourth match on Sunday. 

Lee’s victory comes after he lost three straight games to the Google AI in their historic five-game match that started Wednesday in Seoul.

“This single win is so valuable and I would never exchange this with anything in the world,” Lee said at a press conference right after the victory.

Amid a close game, Lee attacked AlphaGo’s territory on the center zone of the board, but AlphaGo didn’t make defending moves, putting some stones in inexplicable places in other parts of the board. 

Lee didn’t miss the opportunity to take advantage of the opponent’s mistakes. He continued attacking the center zone and secured more territory. 

Even after that, AlphaGo made some clumsy moves until it finally admitted its first defeat to Lee. 

“AlphaGo seems less trained in losing situations. Lee has won the match with his own aggressive and unconventional style of play,” said a commentator at the end of the match. 

With the latest victory, Lee became the first professional Go player to win against AlphaGo. In October, the AI took a 5-0 victory against Fan Hui, the first professional Go player to play with it. 

Apart from the records of the five matches, Google has not revealed any information about AlphaGo’s old matches. The AI has honed itself by studying old matches and training through simulated games. 

Lee and AlphaGo have only one more match planned on Tuesday. Even if Lee wins again, the $1 million prize will be given to AlphaGo, which has already won three matches. 

On Sunday, South Koreans were also elated over the first human victory in the highly-publicized human-versus-AI Go match. 

“I wasn’t that hopeful of the fourth match since Lee already lost three times. I am so surprised that he defeated AlphaGo. I think Lee is likely to be the only human to win Alphago as it will continue evolving,” said a 59-year-old professor surnamed Kim in Seoul.

Some citizens also gave their own versions of how the game had played out and will end eventually.

“I think AlphaGo deliberately lost the game. I heard the machine has the ability to learn through trial and error. I know it sounds a bit scary, but I have watched how smart the machine is. I hope this is not the case,” said 29-year-old Kim Jin-won. 

The South Korean public, along with the rest of the world, have been watching closely the ongoing battle, expressing regret when Lee lost and expressing hope for the remaining games.

Avid Go player 60-year-old Yeo Yeong-soo said, “It seems AlphaGo may not have any emotion but at least has some manners.”

Political leaders including Saenuri Party chairman Kim Moo-sung and The Minjoo Party of Korea interim leader Kim Jong-in also joined the fray, attending the first match held last week.

By Lee Ji-yoon (jylee@heraldcorp.com)

Lee Hyun-jeong and Yeo Jun-suk contributed to this report.--Ed.