Monaco failed to convert three match points before dropping the second set to Hernych. The Argentinian then led 5-2 in the third but Hernych fought back to 5-5 before Monaco clinched the win.
In just his second appearance on the ATP tour this year, Sweden's Joachim Johansson relied on his powerful serve to beat Australia's Peter Luczak 7-5, 6-2.
Simon Greul beat fellow German Benjamin Becker 2-6, 6-3, 7-5 to advance to the second round. He will face Spaniard Guillermo Garcia-Lopez, who beat compatriot Alberto Martin 6-3, 6-2.
Argentina's Leonardo Mayer also advanced after defeating seventh-seeded Albert Montanes of Spain 6-4, 6-3.
La Liga round-upWawrinka, Greul advance at Japan Open
