Why face McCann, a six-time All-Star who hit a three-run homer earlier in the game, when the Nationals instead could face Freeman, a rookie who didn't have a hit in the game at that point?
"I hadn't been in that situation very often, so when they walked Mac, I got a little extra excited," Freeman said. "I wouldn't pitch to him, either."
Mattheus (2-1) walked pinch-hitter Brooks Conrad to start the bottom of the ninth. Jordan Schafer sacrificed Conrad to second before Conrad was thrown out at third on Prado's grounder to shortstop Ian Desmond. Prado stole second base before the walk to McCann.
Freeman lined a 1-0 pitch into right field and was soon in the middle of a celebration between first and second base.
Like Freeman, Braves manager Fredi Gonzalez also said walking McCann was the smart move.
"Pick your poison. I think they made a pretty good decision, but Freddie made them pay," Gonzalez said.
Washington left-hander Tom Gorzelanny left the game with a twisted right ankle in the third inning. He limped off the field after scoring from first base in a home-plate collision with McCann on Roger Bernadina's triple.
Nationals manager Davey Johnson said he believes Gorzelanny, with the help of an off day on Thursday, can make his next scheduled start against the Dodgers on Friday.
"He's got an extra day, so hopefully he'll be able to make his next start in L.A.," Johnson said.
The Braves led 2-0 in the first inning before trailing 6-2 after four, rallying with five runs in the fifth only to again lose the lead when the Nationals scored two in the sixth.
"It was a fun game to play," Freeman said.
Craig Kimbrel (3-2) gave up a walk and a single in a scoreless ninth.
Danny Espinosa had three hits, including a two-run homer in the sixth that gave Washington an 8-7 lead. Espinosa, who also had a single and a run-scoring triple, drove in Bernadina with his 17th homer off George Sherrill.
McCann's 16th homer was the highlight of Atlanta's five-run fifth.
Making his first start since the All-Star Game, Jair Jurrjens gave up a season-high six runs on eight hits and two walks in five innings. He allowed a combined 11 baserunners in the third and fourth but was allowed to pitch through the Nationals' four-run fourth.
"He just couldn't get his breaking ball over," Gonzalez said.
Jurrjens pitched a perfect fifth before leaving the game for a pinch-hitter in the bottom of the inning. His ERA climbed from 1.87, best in the major leagues, to 2.26, still good enough to lead the NL.
"Just mechanics," Jurrjens said. "I was flying open, leaving pitches up. ... It's just something I need to work on in the next bullpen."
It was the second straight game in which a Braves starting pitcher set a season high for runs allowed. Tommy Hanson gave up five runs in Atlanta's 5-2 loss to the Nationals on Saturday night.
Gorzelanny issued three walks in the Braves' two-run first inning. Gorzelanny and Johnson argued when second base umpire Dana DeMuth called a ball for Gorzelanny putting his pitching hand to his mouth while on the mound.
The call gave Dan Uggla a walk, loading the bases, before Gorzelanny's wild pitch allowed Prado to score from third.
After Gorzelanny scored on Bernadina's third-inning triple, Bernadina kept running and scored on Uggla's wild relay throw for an error. The throw forced McCann to move up the third-base line and set up the collision with Gorzelanny.
The Nationals had six hits in the four-run fourth, including a two-run triple by Desmond. Espinosa added another triple off the top of the wall in center before scoring on Ryan Zimmerman's double.
The Nationals optioned INF Brian Bixler to Triple-A Syracuse after the game. INF Jerry Hairston Jr. could come off the disabled list Monday. ... Clippard has allowed eight homers. ... 3B Chipper Jones, on the 15-day DL following arthroscopic surgery on his right knee July 9, will not accompany the Braves on their four-game series at Colorado. He still hopes to come off the DL the first day he is eligible, on July 24. ... Braves rookie Wilkin Ramirez made his second straight start and had a double.