Vitali Klitschko works with his trainer and mentor Fritz Sdunek. Photo by Alexander Hassenstein / Bongarts

Fritz Sdunek, who trained the heavyweight champion Klitschko brothers, Vitali and Wladimir, died of a heart attack on Monday at the age of 67, according to The Associated Press.

News Agency DPA cited Thomas Puetz, head of the Federation of German Professional Boxers, in regard to the circumstances of Sdunek’s passing. “The boxing world has lost a great personality,” said Puetz, according to the AP.

Sdunek, a former amateur boxer who reportedly won 99 of 129 bouts before becoming a coach to amateur and professional boxers, is one of the most successful German trainers of all time, having guided several fighters to major world titles and Olympic medals. The native of Lussow, Germany was the career-long trainer of Vitali Klitschko, a three-time heavyweight titleholder and former RING champion who vacated his WBC belt last year to pursue a political career in his native Ukraine.

“I just found out only a few hours ago. That’s was very unfortunate news,” said Tom Loeffler of K2 Promotions, the Klitschko’s promotional company. “Fritz trained Vitali the whole time and was one of the best trainers ever who had among the most world champions ever. He’s was a really good guy, an excellent trainer and a great man.”

Sdunek also worked with RING heavyweight champion Wladimir Klitschko, guiding the younger sibling to the WBO belt in October 2000, four-time middleweight titlewinner Felix Sturm, former lineal light heavyweight champ Dariusz Michalczewski, former WBC cruiserweight beltholder Juan Carlos Gomez, former WBO 175-pound and WBC cruiserweight beltholder titleholder Zsolt Erdei, Olympic gold medalists Andreas Zuelow and Muhammad Abdullaev, among many other professional and amateur standouts.

“Words cannot express the pain,” wrote Sturm on his Twitter account. “Fritz, I’ll never forget you.”