The Braves and Ervin Santana had a 6-1 lead after four innings Monday against a Red Sox team that was mired in a 10-game losing streak and hadn’t scored six runs once during that skid.

And the Braves lost.

In this maddeningly unpredictable Braves season, they blew a five-run lead in one inning and then watched the Red Sox take advantage of mistakes late in an 8-6 win to start a two-game series at Turner Field.

Santana was charged with five hits, six runs and three walks in five innings, the third consecutive start in which he’s allowed at least five runs.

David Ortiz’s three-run homer off Santana capped a five-run fifth inning that brought the Red Sox even with the Braves, who could’ve been excused for thinking they had scored enough after consecutive three-run innings in the third and fourth gave them a five-run lead.

After all, Santana had looked like himself again to that point of the game, and the Red Sox had batted .212 and totaled 26 runs over 10 games before Monday.

But the bottom fell out on Santana in the fifth, and after a 1-hour, 26-minute rain delay in the sixth the Red Sox quickly moved ahead.

The Braves wasted a scoring opportunity immediately after the delay when Justin Upton walked to start the sixth and Chris Johnson grounded into a double for the 10th time, the most in the National League.

Braves rookie left-hander Ian Thomas started the seventh inning and after not covering first base on an infield hit, walked the next two batters. Ortiz’s bases-loaded sacrifice fly gave the Red Sox their first lead, and A.J. Pierzynski’s single pushed it to 8-6 before Thomas was replaced.

After going 4-0 with a 1.99 ERA in his first six starts for the Braves, Santana is 0-2 with a 9.00 ERA in his past three while allowing 24 hits, 17 runs and eight walks in 17 innings.

Red Sox starter Clay Buchholz had almost as many walks (eight) as outs recorded (nine), but the Braves let him off the hook a couple of times. Before Monday, the last Red Sox pitcher to walk eight in as few as three innings was Lefty O’Doul in 1923.

Buchholz was charged with six runs and left after failing to retire any of the first three batters in the fourth inning.

Legions of Red Sox fans turned out on a warm, sunny afternoon, with estimates ranging from 30-50 percent of a large Memorial Day crowd cheering for the visiting team. The Red Sox fans came to see the Old Town team, but for the first four innings that team looked bad.

From shortstop Xander Bogaerts’ throw wide to first base in the three-run third inning, which creaky-kneed Ortiz wasn’t limber enough to stretch for or quick enough to field and get back to the base in time to make a play; to a grounder that bounced past third baseman Brock Holt for an RBI double in the fourth, to Buchholz’s complete lack of command.

Then came the fifth inning, when the flames of Santana’s past couple of starts were rekindled in a rally that began with a two-out walk by pinch-hitter Daniel Nava and got fully stoked by a mighty swing from Ortiz — who is, after all, paid a lot to hit and not play defense.

Between Nava and Big Papi there was a Holt double, a Bogaerts walk and Dustin Pedroia’s two-run single that cut the lead to 6-3. With the crowd now roaring – Red Sox fans cheering wildly, Braves fans countering – in stepped Ortiz, who was 11-for-31 with three homers in previous matchups with Santana.

Braves manager Fredi Gonzalez left Santana in to face him, and Santana threw a first-pitch changeup before leaving a 1-and-0 slider thigh-high over the outer half of the plate. Ortiz drove it over the center-field fence for his 12th homer of the season and fourth of his career off Santana, whose afternoon had unraveled in alarming fashion.

In a span of five plate appearances, he had gone from 6-1 lead with two out and none on, to scored tied. Rather remarkably, Buchholz had become a mere footnote.

Buchholz walked Jason Heyward to start the fourth inning, after also walking him to start the first and third innings. B.J. Upton followed with an RBI double in the fourth before Freddie Freeman walked to end Buchholz’s off-target afternoon.

Justin Upton greeted reliever Burke Badenhop with a hot grounder that shot past Holt for a double to push the lead to 5-1. One out later, Andrelton Simmons singled to drive in another run.

There still were two runners on with one out, but Ramiro Pena was called out on batter’s interference and Gerald Laird grounded out to end the inning.

The Braves also wasted a scoring chance in the first inning after Buchholz walked two of the first three batters and fell behind 0-and-2 against Justin Upton with one out. Upton ended up striking out on five pitches including a called third strike, and Johnson flied out to end the inning.

Boston took a 1-0 lead in the third after center fielder B.J. Upton got turned around on Grady Sizemore’s leadoff triple. The Braves answered with three runs in the bottom of the inning, after Heyward and Freeman both walked again and Justin Upton drove them in with a double. After an infield single and another walk, Laird lined a bases-loaded single off Buchholz’s leg to push the lead to 3-1.