Radja Nainggolan fired Inter into the lead with a stunner after just seven minutes, as the champions looked sluggish after claiming the title last week.
Juve were sluggish for most of the first half, but eventually levelled through Ronaldo.
The Portuguese played a ‘one-two’ with Juan Cuadrado, before firing home left-footed for his 600th goal of an outstanding career.
Ronaldo scored most of those goals (450) for Real Madrid.
