A master player of the game Go has won his first match against a Google computer program, after losing three in a row in a best-of-five competition.
Lee Se-dol, one of the world's top players, said his win against AlphaGo was "invaluable".
The Chinese board game is considered to be a much more complex challenge for a computer than chess, and AlphaGo's wins were seen as a landmark moment for artificial intelligence.
A fifth game will be played on Tuesday.
Go is a game of two players who take turns putting black or white stones on a 19-by-19 grid. Players win by surrounding their opponents pieces with their own.
Commentator Michael Redmond said AlphaGo had been playing well up until the middle of the game, but at move 78, Mr Lee played brilliantly.
Speaking after his victory, Mr Lee said: "I've never been congratulated so much because I've won one game."
Google representatives said the defeat was "very valuable" for AlphaGo, as it identified a problem which they could now try to fix.
In the first game of the series, AlphaGo triumphed by a very narrow margin - Mr Lee had led for most of the match, but AlphaGo managed to build up a strong lead in its closing stages.
After losing the second match to Deep Mind, Lee Se-dol said he was "speechless" adding that the AlphaGo machine played a "nearly perfect game".
In the third game commentators said that Lee Se-dol had brought his "top game" but that AlphaGo had won "in great style".
The AlphaGo system was developed by British computer company DeepMind which was bought by Google in 2014.
It has built up its expertise by studying older games and teasing out patterns of play.
DeepMind chief executive Demis Hassabis said AlphaGo "played itself, different versions of itself, millions and millions of times and each time got incrementally slightly better".
"It learns from its mistakes," he told the BBC.
What is Go?
Go is thought to date back to several thousand years ago in China.
Using black-and-white stones on a grid, players gain the upper hand by surrounding their opponents pieces with their own.
The rules are simpler than those of chess, but a player typically has a choice of 200 moves, compared with about 20 in chess - there are more possible positions in Go than atoms in the universe, according to DeepMind's team.
It can be very difficult to determine who is winning, and many of the top human players rely on instinct.
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