Huesler secured a 6-4, 7-6 (10-8) victory in the Bulgarian capital after one hour and 42 minutes on court.
"I never really expected it. I never won an ATP event and now I am here. I am playing great tennis the whole week and I'm just really thrilled," Huesler said.
"I played a great, great match, for sure. I am completely taken aback. Just tried to play tennis. I am lost for words. It was a great match from my part as well. I just tried to stay focus and keep calm and I am really, really happy that I am able to make it through."