The Nationals' bullpen allowed only two hits and a walk in four scoreless innings with three strikeouts. Drew Storen had two of the strikeouts in a 1-2-3 fifth -- his fourth straight scoreless outing.
Porcello became the first Tigers starter to work five innings, and he allowed only three singles. Not only did he stretch his streak of innings without a walk to 13, he didn't reach a three-ball count to any of the 18 batters he faced. He threw 42 of his 60 pitches for strikes.
All four of Porcello's strikeouts came against left-handed hitters, a group that hit .325 against him last season. Three of the strikeouts were called third strikes, including Harper to lead off the fourth, and inside fastballs on Steve Lombardozzi and Chad Tracy.
Nationals starter Dan Haren did his best to keep pace, and likely would have if not for a couple of misplays behind him in the second inning. Left fielder Tyler Moore lost Prince Fielder's fly ball in the sun for a leadoff single before Alex Avila pulled a ground ball under first baseman Chris Marrero's glove three batters later to drive in Fielder and give the Tigers a 1-0 lead.
Don Kelly followed with an RBI double down the right-field line to complete the damage off Haren, who allowed five hits in four innings with two strikeouts.
The Nationals' run came in the seventh when Carlos Rivero scored on a Matt Skole sacrifice fly off Duane Below. Detroit's bullpen candidates Brayan Villarreal, who struck out the side in the eighth, and Luis Marte shut it down from there.
Andy Dirks had the lone multi-hit game, going 2-for-3 with a double.
Up next: What do Spring Training numbers mean to a power-precision pitcher? Not that much. Find out more about that sort of thing on Monday, as Stephen Strasburg makes his fourth start of 2013 Spring Training and the 13th spring start of his career. He faces the Braves at home at 1:05 p.m. ET. The game can be heard live on Gameday Audio.
Strasburg's career Spring Training numbers belie his brilliance in games of genuine consequence -- 2-5 with a 3.92 ERA.
Pat Maholm is scheduled to start for the Braves.
Jason Beck is a reporter for MLB.com. Read Beck's Blog and follow him on Twitter @beckjason. This story was not subject to the approval of Major League Baseball or its clubs.