Hm, I guess just the Croatians would have a few words about that...

No, Serbian resistance didn't stop the Northern penetration of Ottoman forces. The Ottoman Empire beat Serbia in Kosovo, and later went on to beat and conquer a third of the Kingdom of Hungary (whose king was also king of Croatia from 1102). However, what's true is that part of the border was the mountainous region that is the border of modern-day Croatia and Bosnia, and many Serbian refugees settled there, also serving as 'buffer people' who'd fight the Ottomans - the Krajnas were a result of this. However, resistance was provided by all the peoples under the Habsburgs along the border, as well as mercenaries from further West, and the Polish-Lithuanian Commonwealth further North and East. What's more, the Ottomans also fought the aging Venetian empire, which ruled much of the Adriatic Coast.

_{*Lunatic*, n.
One whose delusions are out of fashion.}