AMNA NAWAZ: A pioneer in the world of sports has died.

Jason Collins was the first openly gay athlete to play in one of the four major American sports leagues.

Today, tributes are pouring in for a man remembered as a beloved friend, a fierce competitor, and a tireless advocate for equality.

He was a barrier-breaking basketball pro whose bravery inspired a generation.

ANNOUNCER: Jason Collins becomes the first openly gay athlete to play in any of this country's four major professional sports.

AMNA NAWAZ: Jason Collins, a 7-footer known for his hustle defense, played 13 seasons in the NBA for six different teams.

But it was this 2013 essay he wrote for "Sports Illustrated" that made history, where he announced in the first three sentences: "I'm a 34-year-old NBA center.

I'm Black.

And I'm gay."

The country was still two years away from legalizing gay marriage, and the revelation sent shockwaves through professional sports.

Collins told ABC News that day he was at peace.

JASON COLLINS, Former NBA Player: I think the country is ready for supporting an openly gay basketball player.

GEORGE STEPHANOPOULOS, ABC News: Twelve hours later, how does it feel?

JASON COLLINS: It's incredible.

You just try to live an honest, genuine life, and next thing you have the president calling you.

GEORGE STEPHANOPOULOS: What did he say?

JASON COLLINS: He was incredibly supportive.

AMNA NAWAZ: Despite that high-profile support, in the macho world of the NBA, the reaction was mixed.

Some responded with homophobia.

Others, like Kobe Bryant, who just two years before had been fined by the league for using an anti-gay slur on the court, praised him, tweeting: "Proud of Jason Collins.

Don't suffocate who you are because of the ignorance of others."

L.Z.

GRANDERSON, The Los Angeles Times: For Jason to come out in 2013, pre-marriage equality, we're talking about a hostile culture.

It's easy for us to forget what we were like as a nation, but we were not kind, and the NBA was not kind.

AMNA NAWAZ: Sportswriter L.Z.

Granderson covered Collins' career and was a friend.

L.Z.

GRANDERSON: It was a starting center for a team that was in the NBA Finals.

You don't get to be the starting center of a team that good unless you are excellent at what you do.

And Jason Collins was an excellent basketball player.

The league was forced to confront its internal homophobia within its organization and say, there's no way we can justify keeping out a veteran player who has been excellent in his career and caused no problems without looking like we're homophobic.

AMNA NAWAZ: After retirement in 2014, Collins became an ambassador for the NBA.

Last night, NBA all-star Jason Kidd, who played with and later coached Collins, called him a pioneer.

"He had courage like you have never seen," Kidd wrote.

"Those who knew him were blessed to call him a friend.

You are already missed, my brother.

Rest in power."

L.Z.

GRANDERSON: You know, I was blessed to be at his wedding.

And the day before, I spent time with his uncle, who he has talked about before.

His uncle came out first in the family and sort of took the brunt of the shock of it all from the family.

So when Jason came out, he's remarked that it was a lot easier because of his uncle.

We have come a long way as a nation.

And Jason was a major step in that progress in terms of us coming a long way.

AMNA NAWAZ: Collins announced last year he was undergoing treatment for stage four glioblastoma, an aggressive form of brain cancer.

Granderson remembers his friend as much more than an athlete.

L.Z.

GRANDERSON: So, in addition to being a true competitor in the highest sense, And a professional in the highest sense, he was a wonderful friend.

He was a wonderful brother, husband, son, uncle.

And he was a wonderful friend.

And I'm going to miss him a lot.

The world is going to miss him a lot.

AMNA NAWAZ: Collins tackled his diagnosis with the support of his friends and family and the same courage he'd shown in his career.

JASON COLLINS: I'm not afraid to break through a wall or try to do everything possible, I think because I'm so surrounded by love and I know that my family is so strong and they will be OK.

AMNA NAWAZ: Jason Collins was 47 years old.