French Open finalist Zverev, on an eight-match winning streak after winning the Paris Masters title earlier in November, had cruised through to the semi-finals in Turin and not had his serve broken across all three of his group matches.

But that was to change against an inspired Fritz. Though he saved his first break-point opportunity, Zverev could only bat a blistering forehand into the net on his second as the American took a 4-2 lead.

Flustered, the German took only a single point off Fritz's serve for the remainder of the opening set.

After a Zverev hold to love in the first game of the second, drama ensued on Fritz's serve when the chair umpire ruled the ball had bounced twice before a swooping Zverev made contact.

The 27-year-old requested a review which, after a lengthy wait greeted by jeers and whistles from the crowd, proved he had reached the ball in time and the point was replayed.

Two games later, Zverev secured the break that would decide the second set as a powerful forehand, while on the back foot, proved too much for Fritz to dink back over the net.

In the deciding set, just three points were dropped on serve across the opening four games before Fritz survived triple break point.

Zverev later weathered a nervy game himself, saving three break points, before failing to convert two of his own at 5-5 after Fritz had let slip a 40-15 lead.

A hold to love from the German forced the tie-break, but it was Fritz who took control from there, taking a point off Zverev at the first opportunity before a stunning cross-court forehand at match point ensured his Tour season would last one more day.

Looking forward to the final, Fritz - who lost to Sinner in the US Open final in September - said: "I trust my game and I trust my level.

"I don't feel anywhere near as uncomfortable in these situations anymore because I've been putting myself in these situations against the top guys at big events a lot lately. I'm really confident in my game."

Meanwhile, Britain's Henry Patten and his Finnish partner Harri Heliovaara were knocked out of the doubles in a tight 7-6 (7-1) 7-6 (7-4) semi-final defeat by the top seeds, Croatia's Mate Pavic and El Salvador's Marcelo Arevalo.