NEW YORK (Reuters) - Shaquille O'Neal formally announced his retirement on Friday, ending a professional NBA career that spanned nearly two decades.

One of the most successful and recognizable players in the sport, O'Neal had revealed his intentions to quit on Twitter two days ago.

The giant center made it an official with a news conference at his expansive Florida home broadcast live on U.S. television.

"I never thought this day would come," the 39-year-old O'Neal said. "Father Time has finally caught up with Shaquille O'Neal."

During his career, O'Neal played for six NBA teams: the Orlando Magic, Los Angeles Lakers, Miami Heat, Phoenix Suns, Cleveland Cavaliers and finally the Boston Celtics.

He won four NBA Championships, three with the Lakers and one with Miami, scored 28,596 points, the fifth highest total ever, and won a string of personal awards, including the NBA's MVP.

He averaged 23.7 points and 10.9 rebounds a game, was named to 15 All-Star teams and was a member of the United States team that won the gold medal at the 1996 Atlanta Olympics.

"Even though I didn't reach my full potential I can look at my mother and father and say, 'We made it'. A poor family from Newark, New Jersey, from the projects, we made it," he said.

Injuries and the ravages of a physical sport inevitably caught up with the seven feet one inches tall, 325 pound (147 kilograms) O'Neal, forcing him to hang up his size 23 sneakers.

But O'Neal's achievements on court were only part of the reason for his mass appeal. His sharp mind and his infectious smile won him as many admirers as any of his monster slam dunks.

O'Neal has university degrees in arts and business and said he has less than a year to go to complete a doctorate.

He has successfully transcended into the world of entertainment, releasing four rap albums, appearing in movies and hosting his own reality television shows.

"I'm the luckiest guy in the world," he said.

It was no surprise that O'Neal, who gave himself a range of nicknames, including 'The Big Daddy', 'Superman' and 'The Diesel', packed his retirement conference with wisecracks and self-mockery.

"I'm going to miss the competition, the camaraderie, the friendship, the fans, joking with the media," O'Neal said. "And I'm really going to miss the free throws."

A notoriously poor free-throw shooter who averaged 52.7 per cent over his career, O'Neal laughed at how his father told him he could have ended up as the highest point scorer of all time if he hadn't missed so many free throws.

In between the gags, there were some moments of reflection.

He spoke warmly about some of the people who had helped him throughout his career and said he had resolved his rift with his former Lakers team mate Kobe Bryant before he was traded to Miami.

"A lot of people on the outside looking in would say Kobe and I had problems," O'Neal said.

"But I was task-driven and the task was to win championships. I pushed Kobe's buttons. Kobe pushed my buttons. We were able to win three out of four.

"A lot of people think we hate each other, but I see Kobe and his beautiful wife and his beautiful daughters all the time and I go to the babies and say hi, it's Uncle Shaq and they say hi, we kiss them on the cheek. It's all about the task."

(Reporting by Julian Lindenl Editing by Alastair Himmer)