Ke'Bryan Hayes had a stunning major league debut for Pittsburgh, hitting a tying home run and an RBI double. Jose Osuna added an RBI double, and Adam Frazier hit an RBI single.
Pittsburgh's Chad Kuhl pitched four innings, allowing four runs, three earned, and six hits, with two walks and three strikeouts.
On the game's second pitch, Happ launched his 10th homer, to right.
After two singles and an error, Kipnis drew a bases-loaded walk in the second to up Chicago's lead to 2-0.
Colin Moran doubled to right to lead off the Pirates' second, moved to third on Hayes' groundout and scored on Frazier's groundout to make it 2-1.
In the fourth, Kipnis' two-out RBI single gave Chicago a 3-1 lead.
The Cubs got three more in the fifth. Kris Bryant led off with a base hit. An out later, Baez doubled in Bryant. Schwarber followed with a two-run homer to center, his 10th, to push it to 6-1.
Jacob Stallings opened the sixth with a single and Moran walked. Hayes roped a double off the wall in left. Frazier followed with an RBI single to make it 5-3. After Osuna's RBI double, Lester was pulled for Colin Rea. Erik Gonzalez's sacrifice fly brought Pittsburgh to within 6-5.
Following a 71-minute rain delay, Hayes led off the bottom of the eighth against Dan Winkler with a homer to center to tie it.
Each team scored in the 10th, on Jason Heyward's sacrifice fly and Anthony Alford's fielder's choice chopper.
(-Field Level Media)