The extravagantly gifted young Italian, Giovanni Guidi, appeared in a duo with trombone virtuoso/wild man Gianluca Petrella. They both play in Enrico Rava’s quintet, and they both pursue their own projects. (Guidi has a beautifully understated, daring new album on ECM, City of Broken Dreams.) The fact that BJF brought them in as a duo may reflect economics more than aesthetics. Given today’s omnipresent budgetary challenges, many jazz festivals have developed a fondness for solo artists and duos. But putting Guidi and Petrella alone together worked brilliantly, because their fearless imaginations were set free. Guidi was definitely not understated in Belgrade. He splashed and crashed across the full width of the keyboard, and Petrella blasted and squealed, and it all became music. It was a rush when they careened together into “Prelude to a Kiss,” smearing and distorting the song’s tenderness into something hip and hilarious and somehow still heartfelt.