International Go champion Lee Sedol notched up his first victory Sunday in a five-game tournament against an artificial intelligence programme, marking a faint hurrah for humanity over machine.
Lee has already lost the tournament, after Google's AlphaGo machine won the first three sets during the last week. Nonetheless, he celebrated after enduring three losses that he had said left him speechless.
"This win is priceless," he said Sunday, a day after apologizing to fans for his previous losses.
Sunday's game lasted more than four and a half hours.
"Lee surprised AlphaGo," said Michael Redmond, who commented on the game as it was transmitted on YouTube. He said it seemed that AlphaGo was still trying to win even after it had been forced into a losing position.
"I would not want to trade this victory against any other," said Lee after the game. Aged 33, he has been a Go professional since age 12 and has 18 international prizes under his belt.
The Go victory has been seen as a massive advance in computer intelligence, as the ancient game has long been considered too complex for machines to master because of the vast variety of possible options available with each move.
The rules of Go, which originates in China, are easy in principle: two players try to conquer spaces with white and black stones on the board, which is divided by vertical and horizontal lines into a grid.
AlphaGo's programmers said Sunday's loss was valuable, since it will give them the option to analyse what happened and make improvements. Developer Demis Hassabis said several times before the match started that AlphaGo is not perfect.
AlphaGo already beat the European Go champion in October, but beating a world champion like Lee is a higher hurdle to meet. Programmers said they had spent the time since the October win improving AlphaGo's capabilities.
The tournament with Lee will continue through Tuesday. AlphaGo's win means that it will take home the prize of 1 million dollars, which will be donated to charity.