(By Jee Heun Kahng & Se Young Lee, Reuters) – Google’s artificial intelligence (AI) program beat South Korean professional Lee Sedol in the ancient board game Go on Tuesday, recovering from Sunday’s loss to win its fourth match in a five-game series.

AlphaGo, developed by Google subsidiary DeepMind, had already clinched a series victory with its third win on Saturday in a result that shocked the world.

The program made history last year by becoming the first machine to beat a human pro player, but 33-year-old Lee was seen a much more formidable opponent.

“One of the most incredible games ever,” DeepMind founder Demis Hassabis tweeted following the match. “To come back from the initial big mistake against Lee Sedol was mind-blowing!”

Go, most popular in countries such as China, South Korea and Japan, involves two contestants moving black and white stones on a square grid, with the aim of seizing the most territory.

The game is perfect for AI researchers because there are simply too many moves for a machine to win by brute-force calculations, which is how IBM’s Deep Blue famously beat former world chess champion Garry Kasparov in 1997.