Kevin Durant was drafted second overall in 2007.
Russell Westbrook was selected with the No. 4 overall pick in 2008.
The two were teammates in Oklahoma City for eight seasons, but now Durant is a member of the Warriors.
"I talked to him. Obviously, it's tough," Durant said of his conversation with Westbrook. "We were teammates for so long and had a lot of great memories. I'm sure he wasn't happy about the decision but he respected it as my friend.
"Our relationship was great, we had great years man -- something that you can never take away from us ... I couldn't consider everybody's feelings at this point...
"I just tried to focus on me but also realize that the relationships that I had, I still wanted those to be -- I'm sure they won't be the same..."
Over the years, Durant constantly came to Westbrook's defense when anybody challenged the point guard's style of play.
Westbrook, who averaged a career-high 28.1 points per game in 2014-15 with Durant sidelined much of the season with a foot injury, is set to become an unrestricted free agent next summer.
Some in the media have speculated that part of the reason Durant left Oklahoma City was because of Westbrook's uncertain future with the Thunder.
Durant announced his decision on Monday morning at 8:38am PDT. Westbrook hasn't spoken publicly about Durant's departure yet.
"I still wanted to go out there and leave with some class and some dignity," Durant added. "Hopefully I did, and hopefully he understands. I'm sure we'll talk again."