Sixth seed Haase won the ATP 250 event in 2011 and 2012, but fell at the first hurdle this time around as Italian Lorenzi claimed a 6-2 6-3 success.
Lorenzi took full advantage of an under-par serving display from the Dutchman, breaking serve four times in a comfortable victory.
He will now face Pablo Andujar in the second round after the Spaniard continued his recent good form on the clay in Austria.
Andujar won the Swiss Open in Gstaad last Sunday and carried that momentum into his clash with Jurgen Melzer, progressing in straightforward fashion 6-3 6-4.
Juan Monaco, defeated by Andujar in the final in Gstaad, is also through following a 0-6 6-2 6-1 triumph over Kazakhstan's Andrey Golubev.
After succumbing to a whitewash in the opening set, Argentine world number 81 Monaco displayed stunning resolve to fightback and reach the next round as Golubev capitulated.
Monaco's reward is a meeting with Finland's Jarkko Nieminen after the Finnish seventh seed overcame Russian Teymuraz Gabashvili 6-4 6-4.
Elsewhere, wildcard David Goffin lost just four points on his first serve as he beat Kenny de Schepper 6-2 6-4, while promising 17-year-old German Alexander Zverev was hammered 6-1 6-0 by Argentina's Diego Schwartzman.