Drew Storen returned to the mound less than a month ago after missing the first 89 games of the season. He finally hit a rough patch.
Storen, who had 43 saves for Washington last season, was hit around for four runs on three hits while getting just one out in the eighth. The San Francisco Giants used the rally to break open a tight game en route to a 6-1 victory over the Nationals on Tuesday night.
He allowed two runs in his previous 13 appearances after recovering from bone chips.
"I fell behind in the count and tried to make the adjustments," Storen said. "You can't fall behind against these guys. To get back in the groove takes time and you can learn from every outing."
Madison Bumgarner pitched a five-hitter and Brandon Belt had three hits and three RBIs for the Giants.
Bumgarner (13-7) struck out six and walked one in his second complete game this season.
"He dominated with his fastball," the Nationals' Tyler Moore said. "He was good with his off-speed, too. He was locating good."
Adam LaRoche drove in a run for the Nationals, who still have won 11 of 14.
NL ERA leader Jordan Zimmermann (9-7) had his six-game winning streak come to an end after allowing two runs on eight hits. He walked two and struck out four.
"I didn't have my best stuff that's for sure," Zimmermann said. "I was missing here and there a little bit. I just wasn't getting the misses."
Zimmermann has allowed four or fewer earned runs in all 24 starts this season, including 18 of two or less.
"I thought Jordan threw the ball good," Nationals' manager Davey Johnson said. "I've seen him throw better, but Bumgarner was just too tough."
Bumgarner lowered his NL-leading home ERA to 1.85 with his second complete game of the season.
Belt also singled home a run in the sixth. The ball hit high off the wall and bounced right to Jayson Werth, who made a strong throw that nailed Belt at second.
Werth tripled and scored on LaRoche's single in the seventh for the Nationals.
NOTES: RHP Stephen Strasburg (13-5, 2.90) pitches for the Nationals on Wednesday. He leads the NL with 11.2 strikeouts per nine innings. ... Nationals INF Ian Desmond (left oblique strain) could be activated as early as Friday. ... Washington OF Michael Morse was out of the lineup with a jammed right thumb.