A master player of the Chinese strategy game Go has decided to retire, due to the rise of artificial intelligence that "cannot be defeated".
Lee Se-dol is the only human to ever beat the AlphaGo software developed by Google's sister company Deepmind.
In 2016, he took part in a five-match showdown against AlphaGo, losing four times but beating the computer once.
The South Korean said he had decided to retire after realising: "I'm not at the top even if I become the number one."
"There is an entity that cannot be defeated," the 18-time world Go champion told South Korea's Yonhap news agency.
Lee Se-dol is considered to be one of the greatest Go players of the modern era.
The 36-year-old former world champion started playing at the age of five, and turned pro just seven years later.
His defeat by the AlphaGo software was seen as a landmark moment for artificial intelligence.
AlphaGo was developed by Deepmind, which is owned by Google's parent company Alphabet.
"On behalf of the whole AlphaGo team at DeepMind, I'd like to congratulate Lee Se-dol for his legendary decade at the top of the game, and wish him the very best for the future," said Demis Hassabis, chief executive and co-founder of Deepmind.
"During the AlphaGo matches, he demonstrated true warrior spirit and kept us on the edges of our seats to the very end."
Go originated in China 3,000 years ago, and has been played for centuries.
Two players take turns putting black or white stones on a 19-by-19 grid. Players win by taking control of the most territory on the board.
The board game is considered to be a much more complex challenge for a computer than chess.
A player typically has a choice of 200 moves, compared with about 20 in chess - and there are more possible positions in Go than atoms in the universe, according to researchers.
It can be very difficult to determine who is winning, and many of the top human players rely on instinct.
Deepmind hopes that the development of AlphaGo will lead to "similar techniques" that can be "applied to other structured problems, such as protein folding, reducing energy consumption or searching for revolutionary new materials".
Despite his retirement, Lee Se-dol is due to play against another AI system in December.
He will play against HanDol, a program developed by South Korea's NHN Entertainment Corp, which has already defeated the country's top five Go players.
Lee will be given an advantage of two stones in the first game, but suspects he will lose.
"Even with a two-stone advantage, I feel like I will lose the first game to HanDol. These days, I don't follow Go news. I wanted to play comfortably against HanDol as I have already retired, though I will do my best," he said.