The Capitals evened their Eastern Conference quarterfinal appearance at 2-2 and will try to take the lead against the defending Stanley Cup champions Saturday in Boston.
Holtby was pressed into service for the playoffs after both Tomas Vokoun and Michal Neuvirth suffered late-season injuries. Holtby appeared in only seven regular-season games, but has been in goal for all four of Washington's contests in the playoffs.
Holtby faced more than twice as many shots Thursday as did last year's Stanley Cup hero Tim Thomas, but allowed only a Rich Peverley goal with 6:48 left in the opening period. That score tied the contest and Alexander Semin broke the deadlock on the power play with 1:17 to go in the second.
Holtby turned aside 13 shots in the final period. Thomas made 19 saves for the game.
Washington's first goal came on a slap shot from Marcus Johansson 1:22 into the contest.
The Capitals played without center Nicklas Backstrom, who was suspended for cross-checking Peverley at the end of the third game of the series.