"After being able to notify my three children personally and their mother I can inform you of my illness," Ballesteros said. "After exhaustive checks at the hospital they have detected a brain tumour."
Ballesteros will undergo a biopsy on Tuesday after which doctors will decide how to proceed.
The 51-year-old Spaniard was taken to Madrid's La Paz hospital on Monday after collapsing unconscious. Hospital officials then released a statement on Friday confirming that Ballesteros had suffered "a partial epileptic fit" but said he had left intensive care and was in "a stable condition".
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"Throughout my career I have been one of the best at overcoming obstacles on a golf course. Now I want to be the best, taking on the toughest match of my life," he added. "I have always felt solidarity with those facing illnesses, many who were much older than me.
"I want to recall their bravery, belief, calmness, confidence and great mental strength we need to confront these situations however difficult they may be.
"I want to thank from the very bottom of my heart all those people interested in my state of health with many different types of support."
Two-times US Masters champion Jose Maria Olazabal visited Ballesteros in hospital.
"Aside from the logical concern, Seve was very well. We had a long chat," he said. "I wish him a quick recovery."
Another Spanish golfer Miguel Angel Jimenez, who like Olazabal had been competing at the Madrid Masters said: "Seve, we love you. Everyone hopes you recover as soon as possible and we look forward to seeing you again soon."
Ballesteros won the US Masters in 1980 and 1983 and was the Open champion in 1979, 1984 and 1988.
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