Agassi and Graf tell TMZ Sports ... "Stefanie and I are saddened at the loss of our friend. He was one of the kindest, most generous people we have ever known."
Andre Agassi shared some hysterical moments with Robin Williams over the the years -- and tonight he and Steffi Graf are thanking him for his big-hearted efforts to support their charity.
Robin often donated items to Agassi's charity auctions ... going to great lengths to raise money for the Agassi Foundation even when he wasn't able to attend the event.
Andre and Steffi added, "Our prayers are with his family and closest friends during this very sad and difficult time. Today the world has lost a beautiful soul."
Back in 1998 at UCLA ... Robin, Andre and Billy Crystal had the crowd howling at a charity tennis tournaments. Great moment.