Go grandmaster Lee Sedol of South Korea and Google's computer program AlphaGo on Tuesday kept a tight race around the middle of the final match of the best-of-five series at the ancient Chinese board game.
The "match of the century" began at 1 p.m. (0400 GMT) at Four Seasons hotel in Seoul.
The final winner was already determined as Lee lost the first three games of the historic five-game match, but the human Go champion defeated the Go-playing artificial intelligence (AI) in the fourth match, keeping the last remaining pride of professional Go players.
About two and a half hours into the final match, Lee kept a tight game against AlphaGo, a computer program developed by Google's London-based AI subsidiary DeepMind.
AlphaGo attacked the right center by seeking to build a large territory there, while Lee built a territory in the lower-right side of the board according to his strategy which reportedly occupies more areas in an early phase.
In the fourth match where Lee won his first victory after three straight losses, AlphaGo made bad moves after Lee began to dominate in building a territory.
Lee's first victory over AlphaGo indicated that AI hasn't surpassed humans completely in Go, which had been regarded as the last game humans can dominate over machines due to its complex, intuitive and creative nature.
Playing black, Lee put his first two stones right beside flower spots in the right side, while AlphaGo placed its first two white stones in flowers in the left side.
After his first win on Sunday, Lee offered to play black stones, with which he believed AlphaGo displayed a relatively weak play.
AlphaGo boasts of a deep learning capability to learn for itself and discover new strategies by playing games against itself and adjusting neural networks based on a trial-and-error process known as reinforcement learning.
Lee, 33, is regarded as one of the greatest Go players in the world as he won 18 world championships for 21 years of his professional career. He recorded a winning rate of about 70 percent with 47 victories in professional matches.
Go, known as Weiqi in China and Baduk in South Korea, originated from China thousands of years ago. It involves two players who take turns putting white and black stones on a grid of 19 lines by 19 lines. One can win an opponent when gaining more territory on the grid. One can remove stones of the opponent by surrounding the pieces.