Mastnak wins silver at snowboard worlds
Coming second in the qualifying, the 28-year-old former youth world champion was pipped to the gold by Russia's Dmitry Loginov after making a mistake at the third gate in the finals.
He beat Germany's Elias Huber in the best of 16, Austria's Lukas Mathies in the quarter-finals and Germany's Stefan Baumaister in the semis.
"This is super, I'm happy, although with a bit of a lump in my throat. I went for gold. I did great, I believe I'm fit to beat Loginov, who was exceptional and above all reliable today. It didn't add up in the end, though," Mastnak said.
He regretted making the mistake that cost him gold, blaming it on poor visibility. "I didn't see where I went, don't know what happened then, I ended up in the snow," he commented on his final run.
"I've attained my goal, so now I'll be perfectly relaxed. The slalom course suits me well, the steep part turning into a gentle slope. It's tough though, and there's the weather conditions," he said.
Slovenia's Olympic champion Žan Košir finished fifth in Monday's parallel giant slalom.