New York: After getting drubbed by Google-run AI programme "AlphaGo" in three straight matches of Go -- a complex Chinese board game that is considered the "quintessential unsolved problem" for machine intelligence -- 18-time world champion Lee Se-dol finally defeated the artificial intelligence (AI) programme on Monday.
The 33-year-old legendary champion defeated the AI programme in the fourth match of the five-match series currently being held in Seoul, South Korea, The Verge reported.
AlphaGo is now 3-1 up in the series with a professional record and of 9-1, including the 5-0 win against European champion Fan Hui earlier this year.
Se-dol is a South Korean professional Go player. As of February 2016, he ranked second in international titles, behind only Lee Chang-ho.
Lee's first win came after an engrossing game where AlphaGo played some baffling moves, prompting commentators to wonder whether they were mistakes or - as we've often seen this week - just unusual strategies that would come good in the end despite the inscrutable approach, the report said.
The tournament titled "Google Deepmind Challenge match" started on March 8 and will conclude on March 15. The last game will be played on March 15.
DeepMind founder Demis Hassabis said that this time AlphaGo really did make mistakes.
"The AI thought it was doing well, but got confused on move 87," Hassabis tweeted, later clarifying that it made a mistake on move 79 but only realised its error by 87.
"Lee Se-dol is an incredible player and he was too strong for AlphaGo today," said Hassabis, adding that the defeat would help DeepMind test the limits of its AI. "For us this loss is very valuable. We're not sure what happened yet."
Meanwhile, Se-dol Lee entered the post-game press conference to rapturous applause.
"I have never been congratulated so much just because I won one game!" he said.
Go -- a game of profound complexity -- is played by more than 40 million people worldwide. The number of possible positions in the game are more than, as they say, the number of atoms in the universe.
In January, "AlphaGo" defeated Fan Hui - the European champion of the game that was developed in China 2,500 years ago.
Reportedly, Facebook is also working on beating Go and its CEO Mark Zuckerberg said that his artificial intelligence (AI) scientists are "getting close".
"The ancient Chinese game of Go is one of the last games where the best human players can still beat the best artificial intelligence players. Scientists have been trying to teach computers to win at Go for 20 years," he wrote in a post in January.
The first game mastered by a computer was noughts and crosses (also known as tic-tac-toe) in 1952. In 1997, IBM's Deep Blue computer famously beat Garry Kasparov at chess.
The game involves players taking turns to place black or white stones on a board, trying to capture the opponent's stones or surround empty space to make points of territory.