France's Jean-Baptiste Grange, who only recently returned to competition following a knee operation which ended his Vancouver Olympic hopes, dominated the first leg and kept his nerve in the decider to take his first win of the season and seventh of his career.
He clocked a time of 1 minute 46.64 seconds for both runs, relegating Sweden's Myhrer to second place by 0.33 seconds, with Croatia's Ivica Kostelic third, 0.97 seconds off the pace.
Grange, who injured his right knee in December and last raced a slalom at Levi in 2009, topped the times in the first run to signal his ambitions: in all of his previous six wins he had won the opening leg.
Italian Cristian Deville had finished the first run 0.44 seconds down on the Frenchman, with Manfred Pranger -- the last remaining Austrian left in contention -- in third half a second back.
However, both Deville and Pranger struggled to produce the kind of run that would have assured a podium pace. Meanwhile, Myhrer produced the best time of the second run to move up from provisional fourth to second.
Thanks to an equally impressive performance, Kostelic managed to move up from sixth to third to grab the last step on the podium.
A day after Marlies Schild won the women's slalom, Levi's Black Run was not a happy hunting ground for her compatriots on Austria's "Wunderteam," including her fiancé Benjamin Raich.
Raich, Reinfried Herbst, who won here last year, and World Cup slalom champion Marcel Hirscher all crashed out in the first run, as did Olympic champion Giuliano Razzoli of Italy.