Legendary tennis coach Nick Bollettieri -- who trained superstars like Serena and Venus Williams, Andre Agassi and Anna Kournikova -- passed away on Sunday. He was 91 years old.
The International Tennis Hall of Famer was a pioneer in the sport ... creating the first year-round, live-in academy in Florida in the '70s, which would eventually become the famous IMG Academy.
Bollettieri coached 10 #1-ranked players over the years ... including Jim Courier, Boris Becker, Maria Sharapova, Monica Seles, Martina Hingis, Jelena Jankovic, and Marcelo Rios, in addition to the Williams sisters and Agassi.
He also trained Chris Evert, who shared a heartfelt goodbye on Twitter.
"Aside from being the greatest coach ever, you were so kind to me, my parents, and my siblings… that meant more to me than anything, you had a big heart and a zest for living…❤️❤️❤️."
Tommy Haas added ... "Thank you for your time, knowledge, commitment, expertise, the willingness to share your skill, your personal interest in mentoring me, and giving me the best opportunity to follow my dreams."
"You were a dreamer and a doer, and a pioneer in our sport, truly one of a kind."