With one out and two men on base in the bottom of the second inning, Harper battled the sun to make a leaping catch against the wall, robbing Brandon Crawford
of an extra-base hit and stripping San Francisco of at least one run.
"Guys are going out there sacrificing, and that was a huge sacrifice for him," said Nats starter Doug Fister, who tossed seven shutout innings. "He's stretching and catching and rolling on the ground. Those kind of things, guys are sacrificing for the team aspect, and that's what we're doing."
Then, with the Nationals leading 3-0 in the seventh, Harper came bolting in on a line drive by Travis Ishikawa and snagged it at his shoestrings with a runner on second.
"It's a difficult outfield to play," Nats manager Matt Williams said. "Sunny field in left on a day game like this, so it's not easy. ... It wasn't really windy early on, but the wind picked up and he made a couple really nice plays. He was great defensively, went up against the wall and made that play with a couple guys on. That could change the game for us."
"Going out there and being able to deal with that sun a little bit, it's very tough," said Harper. "We have that a little bit in D.C. in center, so really had it all year long. It's definitely tough, trying to battle out there. [I was trying to make] some catches and make some plays and not let them score and not let them hit."
In addition to taking away multiple runs from the Giants, Harper also crossed home plate twice. He walked and scored on the error by Bumgarner in the seventh, and then in the top of the ninth, Harper added the cherry on top: a solo home run to right field off Jean Machi to extend Washington's advantage to 4-0.
Harper now has three career postseason homers -- two in this series -- making him the fourth player to hit that many in the playoffs before turning 22. The others: Mickey Mantle, Miguel Cabrera and Andruw Jones.
Thanks in large part to their slugger with talent beyond his years, the Nationals will play again on Tuesday (9 p.m. ET on FOX Sports 1).
"We're going to go out there tomorrow and do what we need to do," Harper said, "and hopefully get some runs up there for Gio [Gonzalez] and get back to D.C. for Game 5."