BOSTON -- Pinch hitter Mike Carp delivered the game-winning hit in the 10th inning, leading the Red Sox to a 4-3 victory over the Chicago White Sox on Thursday, Boston's second walk-off win in as many days.
Conor Gillaspie tied the game with a pinch-hit, two-run homer in the ninth inning off All-Star closer Koji Uehara.
Ronald Belisario (3-6) walked pinch hitter Daniel Nava to begin the 10th, and right fielder Mookie Betts moved the runner to second with a sacrifice bunt. After an intentional walk to shortstop Stephen Drew, Carp's single to left plated Nava.
Andrew Miller (3-5) pitched one inning for the win.
Boston trailed 4-0 in the eighth inning Wednesday before storming back for a 5-4 victory. The Red Sox and White Sox split the four-game series.
Jon Lester tossed seven solid innings and designated hitter David Ortiz delivered a two-run double for the Red Sox, who had just three hits en route to their third win in 10 games.
Chicago shortstop Alexei Ramirez singled off Uehara to begin the top of the ninth, barely beating the throw from first baseman Mike Napoli to a covering Uehara. Ramirez was originally called out before Chicago manager Robin Ventura challenged the play and the call was overturned.
Gillaspie then pinch-hit for right fielder Moises Sierra and belted his third homer in as many games and fourth of the season, driving Uehara's second pitch down the right field line and just inside the foul pole to tie it 3-3.
The homer erased another dominant start by Lester, who continued his sizzling summer by giving up one run on seven hits with no walks and 12 strikeouts.
One of Boston's two All-Star selections, Lester has permitted just five earned runs over his last six starts, a span of 44 2/3 innings in which he owns a 1.01 ERA.
Chicago's Jose Quintana has been nearly as dominant.
The southpaw was perfect through five innings Thursday before sputtering in the sixth, allowing his first earned in 20 2/3 innings and only the second in his last 30 2/3 innings.
Quintana yielded three runs on just two hits over seven innings, striking out seven and walking two.
Quintana retired the first 15 batters he faced before walking Drew and catcher David Ross to begin the sixth. Center fielder Jackie Bradley Jr. followed with Boston's first hit of the game, a single to right that scored Drew and tied it at 1-1.
Ortiz's two-out double put the Red Sox on top 3-1.
Chicago took an early lead when center fielder Adam Eaton led off the game with a single and scored from first on first baseman Jose Abreu's double.
NOTES: The White Sox were trying to win their first four-game series at Fenway Park since 1967. ... Boston manager John Farrell said Christian Vazquez, who made his debut Wednesday, and David Ross would switch off catching every other day until the All-Star break. ... Boston RF Shane Victorino, limited to 21 games this season with back and right hamstring injuries, began a new rehab assignment with short-season Class A Lowell on Thursday night. Farrell said Victorino likely would play two games with Lowell and two with Triple-A Pawtucket prior to the All-Star break. ... Boston RHP John Lackey (9-6, 3.84 ERA) will oppose Houston RHP Scott Feldman (4-5, 3.86) in the opener of a three-game series Friday. ... White Sox RHP Hector Noesi (3-6, 4.90) will face Cleveland RHP Corey Kluber (8-6, 2.86) when the teams begin a three-game set Friday. ... Chicago LHP Chris Sale (8-1, 2.08 ERA) was tabbed as the 34th and final player to the American League roster for the All-Star game after winning the fan vote.