Ballesteros taken in for surgery bbcsportsGolf legend Seve Ballesteros is undergoing a third operation to reduce brain swelling after the discovery of a tumour earlier this month.
The 51-year-old collapsed on 6 October and is in a serious but stable condition at La Paz Hospital in Madrid.
Doctors operated last week, but the Spaniard needed more surgery to relieve pressure on the brain two days later.
Friday's operation to remove the remains of the tumour began at 0930 BST and is expected to last all day.
A hospital statement said Ballesteros has an oligoastrocytoma, which is a tumour composed of two types of brain cell.
The tumour is said to be affecting nerve cells and the spinal cord - making it difficult for surgeons to reach.
BBC Radio 5 Live's Steve Kingstone in Madrid said: "There's no doubt this is very, very serious. What doctors will do today is first of all attempt to remove more of the tumour following the initital surgery last Tuesday.
"They will also try to ease some of the complications that followed last week's surgery, specifically a build up of fluid on the brain, and they've also detected bleeding inside the brain.
"They describe his condition as stable but it's obviously very serious surgery that he's undergoing today."
Ballesteros won 87 titles during his career, including the Open in 1979, 1984 and 1988 and the Masters in 1980 and 1983, and captained Europe to Ryder Cup victory at Valderrama in 1997.
He retired last year following arthritic back and knee problems late in his career, while doctors discovered an irregular heartbeat when he was admitted to hospital in 2007.
All the best Sesv, my thoughts and 
are with you and your family :Jimmie O':
