Horton set up tallies by Krejci in the first period and at the 7:51 mark of the final frame, then added his sixth goal of the playoffs less than 4 minutes later as the Bruins dominated play.
Tuukka Rask stopped all 29 shots he saw for a shutout as fourth-seeded Boston showed few signs of rust following a weeklong layoff since winning its conference semifinal series against the New York Rangers in five games.
Tomas Vokun made 27 stops for the top-seeded Penguins, who will attempt to even the series in Monday's Game 2.
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