Mika Immonen, a two-time world champion pool player from Finland known as the Iceman, who was admired as much for his physical stamina and mental acuity during competitions as he was for his skill at the game, died on Sunday in Manhattan. He was 52.

His death, in a hospital, was confirmed by his friend Alex Masucci, who said the cause was cancer. Immonen had learned in late 2023 that he had Stage 4 colorectal cancer.

For someone who excelled at a sport not known for its athletic requirements, Immonen was adamant about exercising daily and limiting his intake of carbohydrates. He said he believed that the focus he applied to his workouts translated into heightened concentration during competitions, and that any pool player who didn’t exercise wasn’t trying hard enough to improve at the game.

“My favorite expression of his was ‘Mercy is a disease’ — if you feel sorry for your opponent, it’s contagious,” said Jonathan Smith, the house professional pool player at Society Billiards and Bar in Manhattan, where Immonen competed in recent years. “He was a phenomenal shotmaker, and he played at a very fast clip, which made playing him even more intimidating.”

Immonen won a number of U.S. and international tournaments, including the World Pool-Billiard Association’s World Nine-Ball Championship in 2001; back-to-back U.S. Open Nine-Ball Championship titles in 2008 and 2009; and the W.P.A. World 10-Ball Championship in 2009. In 2016, he won the world title in straight pool in New York City against Earl Strickland, his former mentor, who has been called the Michael Jordan of pool.

But the game that elevated Immonen to elite circles was his 2001 win against Ralf Souquet of Germany. Upon sinking the final nine ball, Immonen collapsed on the table and broke into tears while the crowd chanted, “Mika! Mika!”

After that, “everybody knew me,” he said in a 2010 video interview with Jim Murnak, a maker of custom pool-cue cases. “I was like, ‘What is this? I’m famous.’”

The victory bolstered his ego.

“He got such a buzz from winning the world title,” The Evening Standard of London noted in a 2001 profile, “that for a week afterward he answered the telephone with the words: ‘Hello, Mika Immonen, world champion.’”

Immonen exhibited a degree of arrogance that complemented his Iceman moniker, which he gained for his cold stare and steely demeanor at the table.

“They tell me I look mean on TV,” he said in a 2006 interview. “But I just say that’s the way I play. It’s serious business.”

Mika Ilari Immonen was born on Dec. 17, 1972, in London and was named after the best-selling Finnish author Mika Waltari. His father, Ilkka, was a tailor; his mother, Laura Paloheimo, studied interior architecture, but turned her focus to raising Mika, his twin brother and their older sister.

Mika was 3 when his parents divorced. Four years later, his mother moved the children to Helsinki, Finland’s capital.

When he was around 15, a billiards hall and arcade opened two blocks from his home, and he and his brother and their friends started shooting pool as an indoor alternative to playing ice hockey in the bitter cold.

The popular game at the time was kaisa, or karoliina, which was more challenging than American pool because it was played on tables with pockets so tight that there was barely enough room for the balls to enter. “Within a short period of time, I discovered that I had talent,” Immonen said in the 2010 video interview. “I was doing all these shots with very high accuracy.”

About a year later, the pool hall’s owners encouraged him to enter a tournament in a nearby city, and he won. The prize was a ham, which he sold for $100. At 17, he beat the Russian professional player Ashot Potikyan.

Immonen began playing snooker and other games. He competed regularly and often won, using his prize money to travel and play in more competitions. In 1992, when he was 20, he quit smoking and took up running, eventually competing in marathons, which he saw as a pure form of endurance training for life — and, of course, for pool.

“In a sense, you can relate that when you’re playing a game of pool,” he said in the 2010 interview. “Sometimes you have these doubts or stupid ideas in your mind, ‘Hey I don’t feel like doing this now,’ but then you’re like, ‘Come on man, keep fighting.’”

In 2014, he was inducted into the Billiard Congress of America Hall of Fame.

Immonen is survived by his mother; his partner, Emily Elizabeth Blair Kerr Keaty; a sister, Rea Paloheimo; and three stepbrothers, Jaakko Nevasto, Mikko Immonen and Christian Olsen. His twin brother, Kari Jalmari Paloheimo, died earlier this year.

Immonen was traveling for tournaments in 2023 when he began feeling sharp pains in his lower back. It wasn’t until he was speaking with a friend in Costa Rica who was an oncologist that he realized he might have cancer, he told Molina Mike on the podcast “Doggin’ It” that December. While receiving chemotherapy treatments in Costa Rica, he said on the podcast, he continued to exercise and play in local tournaments.

During the interview, fans in a chat room shared messages of encouragement, including “sisu,” a Finnish word for resilience.

“It is a word that encapsulates so many things,” Immonen said. “But it’s like this just sheer will and determination and never giving up.”

He added: “I know it sounds like a cliché, but when I come back, I am going to be stronger.”

Ash Wu contributed reporting.