Javier Baez - a 21-year-old from Puerto Rico - hit a home run in the 12th inning at Coors Field to give the Cubs a 6-5 win over the Colorado Rockies, while in Seattle, pitcher Felix Hernandez led the Mariners to a 4-2 triumph against the Braves.
Atlanta again missed the chance to gain ground on the Washington Nationals, who lead the National League's (NL) East division, as Hernandez pitched eight innings for just four hits and one run to register his 12th win of the season.
The Braves scored first at Safeco Field but a four-run inning in the fourth from the hosts ensured Atlanta's losing streak was extended.
Atlanta have not won since July 28 - the longest existing winless run in the MLB - and they remain second in the NL East with a 58-55 win-loss record, three games behind Washington, who lost their second straight match.
The third-placed Miami Marlins moved within 5.5 games of the Braves with a 6-3 victory at the Pittsburgh Pirates.
Miami scored five runs in the eighth inning to overcome the Pirates, while in the capital, the New York Mets crushed the Nationals 6-1.
In Denver, the Rockies led 3-0 after five innings but a home run from Welington Castillo started the Cubs' comeback.
Chicago edged 4-3 ahead midway through the seventh inning but Colorado responded immediately thanks to Nolan Arenado's homer.
Arenado's run forced extra innings and the Cubs thought they had clinched victory when Ryan Sweeney scored Starlin Castro, only for the home side to equalise again.
Finally in the 12th inning, Baez smashed a pitch from Boone Logan over right field to decide the game, with the Puerto Rican infielder becoming an instant hero with Cubs fans.
In the American League (AL) East division's standings, the Baltimore Orioles stretched their lead to five games over the Toronto Blue Jays with a comfortable 9-3 win over the Canadian outfit.
Caleb Joseph, Jonathan Schoop and Chris Davis all cleared the fence for Baltimore, while Colby Rasmus hit a homer for the Blue Jays, as the Orioles took their record to 64-48.
All other AL East teams lost with the New York Yankees going down 4-3 to the Detroit Tigers after 12 innings, while the Boston Red Sox lost 3-2 to the St Louis Cardinals and the Tampa Bay Rays succumbed 3-0 to AL West-leaders the Oakland Athletics.
The Texas Rangers claimed the biggest win of the day with a 16-0 demolition of the Chicago White Sox, while the Kansas City Royals thrashed the Arizona Diamondbacks 12-2 and the Cincinnati Reds trounced the Cleveland Indians 9-2.
The Dodgers won the Los Angeles derby 5-4 over the Angels, while in other results, the Philadelphia Phillies needed 15 innings to overcome the Houston Astros 2-1, the Milwaukee Brewers edged out the San Francisco Giants 4-3 and the Minnesota Twins won 3-1 against the San Diego Padres.