Google DeepMind's AI system, AlphaGo, has won the first of three matches it is playing against the world's number one Go player, Ke Jie.
It follows its historic win against Lee Se-dol last year, described by experts as a breakthrough moment for AI.
The AI won by just half a point in its latest match.
Ke Jie described the AI as "like a god of Go players", while DeepMind co-founder Demis Hassabis thanked him for a hard-fought match.
"It was a such close game, an exciting game and showed how much work Ke Jie put into preparing for the match," said Mr Hassabis in a post-match press conference.
"It was interesting for us to see him using moves from AlphaGo's previous games, and we were intrigued to see how AlphaGo deals with its own strategies used - huge respect to Ke Jie for pushing AlphaGo to its limits."
He added that the ultimate plan for AlphaGo was a wider deployment "in areas of medicine and science".
Of the match, Ke Jie said: "There were some unexpected moves and I was deeply impressed.
"I was quite shocked as there was a move that would never happen in a human-to-human Go match."
He added that AlphaGo was a "wonderful player" and "completely different" from last year.
AlphaGo now has huge influence on the Go circuit, and was star turn at the Future of Go Summit, organised by Google, in China this week.
The second game will take place on Thursday, with the final match on Saturday.
In Go, players take turns placing stones on a 19-by-19 grid, competing to take control of the most territory.
It is considered to be one of the world's most complex games, and is much more challenging for computers than chess.
AlphaGo has built up its expertise by studying older matches and playing thousands of games against itself.