After losing three straight matches, Korean Go world champion Lee Sedol finally broke through Sunday, securing his maiden victory in the fourth match of the five-game series against AlphaGo — the program developed by researchers at Google’s DeepMind division. The victory proves that the computer program, whose development is being hailed as a breakthrough in the field of machine learning and artificial intelligence (AI), is not invincible and that it may still take some time to master the ancient Chinese board game.

According to tweets by DeepMind’s co-founder Demis Hassabis, the program, which learnt to play Go by watching millions of games, did make a few “mistakes” during Sunday’s match.

“AlphaGo thought it was doing well, but got confused on move 87,” Hassabis tweeted. “We are in trouble now.”

“Mistake was on move 79, but AlphaGo only came to that realization on around move 87,” he wrote in a separate tweet.

Go is believed to have been invented in China nearly 2,500 years ago. It’s played by placing black or white stones on a square grid. When a player surrounds any of his opponent’s pieces, they’re captured. The goal of the game is to control at least 50 percent of the board.

While the rules of the game are simpler than those of chess, the overall complexity is much higher, making it, in the words of Hassabis, a much more “intuitive” game.

“Go is a game of profound complexity,” Hassabis recently wrote in a blog post. “This complexity is what makes Go hard for computers to play, and therefore an irresistible challenge to AI researchers, who use games as a testing ground to invent smart, flexible algorithms that can tackle problems, sometimes in ways similar to humans.”

As a result, the ongoing series in Seoul is being widely viewed as an important test of how far AI research has come in its quest to create machines imbued with human, or even superhuman, level of intelligence.

“When it comes to human beings, there is a psychological aspect that one has to also think about,” a visibly downcast Lee said Saturday, after losing his third match. “I do have extensive experience in terms of playing the game of Go, but there was never a case as this as such that I felt this amount of pressure.”

The final match between Lee and AlphaGo will take place Tuesday.