The Slovenian racked up unforced errors in the first set and appeared to be headed for defeat but sharpened her focus in the second. She controlled the match to go up 5-1 in the third but then lost her concentration once again, allowing Johansson to fight back to 5-5.
Hercog recovered by breaking Johansson in the next game and then serving out the match.
"Sometimes nerves kick in too much," Hercog said. "But I'm really glad that I managed to calm myself down in the last two games."
Hercog beat Sweden's Johanna Larsson in the final last year.