Madrids La Paz hospital said the 51-year-old Spaniard was taken to the operating area at 8:30 a.m. local time (2:30 a.m. EDT) and that surgery began two hours later. The operation ' described as one of great complexity ' was expected to continue until late in the evening.
The surgery is aimed at relieving pressure on his brain caused by swelling and bleeding. It will also remove remaining parts of the malignant tumor located in a very deep part of the brain.
The hospital said a statement would be released as soon as doctors report on the operation on the winner of five majors.
Ballesteros tumor is an oligoastrocytoma, which affects cells that cover and protect the nerve cells in the brain and spinal cord.
Ballesteros was admitted to the hospital on Oct. 6 after briefly losing consciousness at Madrids international airport. On Oct. 14, a sizable part of the tumor was taken out.
In a second operation days later, part of his skull was removed to allow room for the swelling brain to expand.
Ballesteros was reported in stable but serious condition prior to Fridays surgery.
Ballesteros, who won three British Opens and two Masters, said in a statement recently from his hospital bed that he faced the hardest challenge of my life.
Ballesteros, winner of a record 50 tournaments on the European Tour, retired last year because of a long history of back pain and has since concentrated on golf course design.
Ballesteros revolutionized European golf. After the Ryder Cup was expanded to include continental Europe in 1979, he helped beat the United States in 1985 to begin two decades of dominance. He also captained Europe to victory in 1997.
Ballesteros and Jose Maria Olazabal made one of the most formidable partnerships in Ryder Cup history, with 11 wins, two losses and two halves.