Physicians at La Paz Hospital in Madrid said Thursday that a part of Ballesteros' skull was removed to relieve the pressure. They said the procedure was not uncommon following operations such as Ballesteros had Wednesday.
Ballesteros underwent about 12 hours of surgery to remove a tumor. After he showed a decreased level of consciousness, doctors Thursday opted for decompressive craniotomy.
Ballesteros collapsed Oct. 6 at a Madrid airport and subsequent examinations indicated a brain tumor. On his Web site in a message dated Sunday, Ballesteros confirmed the existence of the tumor and said, "I want to be the best facing the most difficult match of my life."
Ballesteros won the British Open three times and the Masters twice. His 50 European Tour victories top the all-time list there and he had 87 total tournament victories. He was inducted into the World Golf Hall of Fame in 1999.