Petzschner, ranked 71st and Wimbledon men's doubles champion last year, outsmarted the world number seven and second seed with a mix of powerful serves, sliced crosscourt backhands and drop shots that threw the Czech off balance.
Kohlschreiber eased past world number eight Gael Monfils in straight sets for another upset to set up the grass court tournament's first all-German final for 19 years.
Berdych, favorite since the withdrawal of top seed Roger Federer, struggled in the first set with both players holding serve early on and the German's powerful serve pinning him back.
An audacious drop shot earned Berdych two set points at 5-4, which Petzschner fought off with an ace, and another powerful serve that the Czech sent wide.
The pair traded breaks and wasted a set point each in the tiebreak before the 27-year-old German snatched it when Berdych sunk a forehand into the net.
The second set, however, was a completely different story with Berdych racing to a 4-0 lead as Petzschner suffered one of his dreaded and infamous blackouts.
He recovered in time for the start of the third set to break his opponent and go 5-2 up, clinching a place in his first ATP final since 2008 when Berdych overhit another forehand.
Earlier on Saturday world No. 49 Kohlschreiber knocked out Frenchman Monfils 6-3 6-3 to reach the final for the second time since 2008.
Kohlschreiber, who has struggled this year and has dropped down the pecking order to number 49 in the past few months, needed only an hour and 17 minutes to dispose of the Frenchman.
"I played a lot of slice today and showed courage at just the right points," Kohlschreiber told reporters. "Monfils usually puts on a very good show but today he was not strong enough." Monfils struggled with his serve from the start, handing the German a break in the first game with a double fault and being treated early on for some pain in his right arm. Kohlschreiber broke once more to clinch the first set.
Another break in the second outing was all Kohlschreiber needed as he gave Monfils not a single break chance throughout the match. "Congratulations to Philipp. He played an outstanding match today," said Monfils.
