Davis' homer, his 14th of the season, came shortly after the Red Sox had scored twice off Giants closer Camilo Doval in the top of the ninth, drawing even on a two-run single by Justin Turner.
Despite suffering his third blown save of the season, Doval (3-3) was credited with the win. Jansen (2-5) took the loss.
The dramatic ending came after Sean Manaea had combined with opener Ryan Walker and set-up reliever Tyler Rogers to take a 2-0 lead into the ninth.
Dodgers 3, Reds 2
Max Muncy hit a pair home runs for Los Angeles' only two hits and right-hander Joe Kelly stranded the bases loaded in the sixth in his return to the Dodgers in a victory over visiting Cincinnati.
Rookie right-hander Emmet Sheehan gave up two hits over five scoreless innings as the National League West-leading Dodgers. Right-hander Evan Phillips recorded the final four outs for his 13th save.
Reds right-hander Luke Weaver (2-3) gave up the two Muncy home runs and was effective over six innings. Cincinnati rookie Elly De La Cruz doubled and scored a run as the Reds still maintained a winning record over the Dodgers this season at 3-2 despite the loss.
Padres 4, Rangers 0
Yu Darvish pitched six shutout innings and Ha-Seong Kim capped a four-run second with a two-run single as San Diego blanked visiting Texas.
Darvish gave up three hits and two walks with nine strikeouts to improve to 8-7. Luis Campuano had two hits and drove in a run, and Gary Sanchez also had an RBI.
Rangers starter Martin Perez (8-4) gave up four runs on eight hits and three walks with a strikeout in 5 2/3 innings.
Braves 11, Brewers 5
Ronald Acuna Jr. stole a base to help ignite a six-run first inning, setting a record in the process, as Atlanta cruised past visiting Milwaukee.
When Acuna stole second after a leadoff single, he became the first player in major league history to collect at least 20 home runs and 50 steals before August. The Braves' opening deluge was capped when Eddie Rosario hit a two-run home run -- the first of the team's four homers in the game.
Milwaukee starter Julio Teheran (2-5) was tagged for nine runs in his five innings. Braves starter Bryce Elder (8-2) gave up just one run in seven innings and struck out three.
Athletics 11, Rockies 3
Brent Rooker homered and drove in three runs and Zack Gelof homered and scored three times to help Oakland to a win against Colorado in the second game of their three-game series in Denver.
Seth Brown had three hits and two RBIs, Tyler Soderstrom delivered two hits and an RBI and Ramon Laureano contributed two RBIs for Oakland, which will try for its second sweep of the season on Sunday.
A's starter Paul Blackburn (2-2) allowed two runs and nine hits in six innings. He struck out seven and didn't walk a batter. Rockies starter Chris Flexen (0-5) had six walks, a hit batter and a run-scoring wild pitch in 3 2/3 innings. He was tagged for six runs (five earned) and four hits, two of which left the park.
Astros 17, Rays 4
Hunter Brown carried a shutout bid into the seventh inning and benefited from ample run support as Houston recorded nine extra-base hits and rolled to a victory over visiting Tampa Bay.
Yainer Diaz, Jose Abreu, Jose Altuve, Alex Bregman and Yordan Alvarez all homered for the Astros, the latter three doing so in the bottom of the seventh inning off Rays reserve catcher Rene Pinto. But Houston held a nine-run lead before things got silly in that frame, as Chas McCormick, Diaz and Jake Meyers all posted multi-hit games from the bottom third of the order.
Rays rookie right-hander Taj Bradley (5-7) allowed five runs on three hits and four walks with four strikeouts in 3 1/3 innings.
Blue Jays 6, Angels 1
Alejandro Kirk hit two home runs and had three RBIs, Santiago Espinal added a two-run shot and Toronto defeated visiting Los Angeles.
Toronto right-hander Alek Manoah struck out six and allowed one run, four hits and three walks in 4 1/3 innings. He left after hitting Taylor Ward in the face with a pitch with the bases loaded in the fifth inning. Ward appeared to be bleeding from the face and was removed from the field on a cart.
Angels left-hander Reid Detmers (2-8) allowed two runs, three hits and two walks in 4 2/3 innings. He struck out five.
Tigers 5, Marlins 0
Akil Baddoo slugged a three-run homer to lead Detroit to a win over host Miami.
Riley Greene added a solo blast to power the Tigers. Five Detroit pitchers combined on the shutout, with Joey Wentz (2-9) earning the win with 4 2/3 innings of relief.
Marlins second baseman Luis Arraez, who leads the major leagues with a .381 batting average, was a Miami bright spot, going 2-for-4. Johnny Cueto (0-2) took the loss, allowing four hits, one walk and four runs in six innings.
White Sox 7, Guardians 2
Mike Clevinger pitched five shutout innings in his return from the injured list while Tim Anderson and Andrew Vaughn homered to spark Chicago to a win against visiting Cleveland.
Chicago has won two straight on the heels of a six-game losing streak after seizing control of Saturday's contest in the early going. Anderson connected for a leadoff home run against Guardians starter Logan Allen. It was the first of 12 hits for Chicago, which assured at least a split of the four-game weekend series.
Activated from the IL before the game, Clevinger picked up the victory in his first start since June 14. The right-hander scattered two hits and three strikeouts without issuing a walk to improve to 4-4. Allen (4-4) took the loss, yielding five runs and eight hits in six innings with three walks and four strikeouts.
Yankees 8, Orioles 3
Aaron Judge hit his first home run since June 3 to help New York pick up a victory against host Baltimore.
Giancarlo Stanton and Kyle Higashioka also homered and Isiah Kiner-Falefa drove in three runs. Judge came off the injured list on Friday and played for the first time since that June 3 game in Los Angeles. His 20th home run of the season marked his first hit in the series. Judge, who played as the right fielder, finished 3-for-5 and scored two runs. Yankees starter Clarke Schmidt (7-6) pitched five innings and allowed three runs.
Baltimore starter Tyler Wells (7-6) was gone after 2 2/3 innings in his third straight short outing. He gave up three runs, three hits and three walks on the way to dropping to 0-5 all-time versus the Yankees.
Royals 10, Twins 7
Bobby Witt Jr. homered and drove in three runs to lead Kansas City past visiting Minnesota.
In his first at-bat, Witt picked up where he left off following Friday's walk-off grand slam. He hit his 18th homer in his first at-bat. Michael Massey's leadoff homer into the right field bullpen began a three-run second inning, capped by Witt's RBI single for a 4-0 advantage. Witt added an RBI triple in the seventh, collecting his eighth hit and ninth RBI of the series.
Bailey Ober (6-5) surrendered six runs on 11 hits with five strikeouts and no walks in four innings for the Twins.
Cubs 5, Cardinals 1
Ian Happ and Yan Gomes hit two-run homers as visiting Chicago defeated St. Louis for its eighth straight victory.
The Cubs have won 10 of their last 12 games to surge into the playoff race. The Cardinals have lost seven of their last nine games with trade speculation swirling around them. Chicago starter Jameson Taillon (5-6) allowed one run on two hits and four walks in six innings. Relievers Michael Fulmer, Mark Leiter Jr., Javier Assad and Daniel Palencia closed out the last three innings for the Cubs.
Cardinals starting pitcher Adam Wainwright (3-5) remained stuck on 198 career victories. He induced three double-play grounders, but he allowed four runs on seven hits and three walks in six innings.
Pirates 7, Phillies 6
Endy Rodriguez hit a three-run triple in a four-run fifth inning and had an earlier RBI double as host Pittsburgh downed Philadelphia.
Liover Peguero added a homer and RBI single and Alika Williams had an RBI single for the Pirates. Starter Quinn Priester (2-1) allowed five runs and five hits in five-plus innings, with seven strikeouts and six walks. David Bednar earned a five-out save, his 21st.
Brandon Marsh hit a three-run double, Jake Cave an RBI double and Johan Rojas an RBI single for the Phillies, who had won three straight. Philadelphia starter Aaron Nola (9-7) gave up seven runs, five earned, and nine hits in 4 2/3 innings, with six strikeouts and three walks.
Nationals 11, Mets 6
Riley Adams had three RBIs as Washington beat host New York during a game in which the Mets dealt Max Scherzer to the Texas Rangers.
Dominic Smith, Joey Meneses and Stone Garrett finished with two RBIs apiece for the Nationals. Patrick Corbin (7-11) allowed four runs on six hits and no walks while striking out two over 5 2/3 innings.
Mark Vientos hit a homer and had two RBIs for the Mets. Francisco Lindor hit a two-run homer while Tommy Pham and Francisco Alvarez had a solo shot apiece. Carlos Carrasco (3-5) gave up eight runs (six earned) on eight hits and one walk while striking out two over 2 1/3 innings.
Diamondbacks 4, Mariners 3
Dominic Canzone singled in the tiebreaking run in the bottom of the eighth inning as Arizona rallied to defeat Seattle in Phoenix.
Canzone had two hits and two RBIs as the Diamondbacks recovered from a 3-0 deficit. Scott McGough struck out Julio Rodriguez with runners on the corners in the ninth to end it as Arizona improved to 4-10 since the All-Star break and won for just the sixth time in the past 21 games. Arizona's Kevin Ginkel (4-0) struck out the side in the eighth.
Tom Murphy homered and drove in two runs for the Mariners. Corbin Carroll set up the decisive run by walking against Andres Munoz (2-4), moving to second on a balk and then stealing third. After a groundout to third, Canzone hit a high-chopping single to center to score Carroll.
