Through six innings, Nelson allowed four hits, one walk, while striking out eight Cubs batters.
Jared Hughes (1-0) pitched 1 2/3 innings of scoreless relief.
Milwaukee Brewers' Orlando Arcia (3) and Jonathan Villar react after turning a double play on a ball hit by Chicago Cubs' Ben Zobrist during the 10th inning of a baseball game Friday, April 7, 2017, in Milwaukee. It appeared he was trying to convince Joe Maddon to leave him in the game, but wisely, Joe chose to have Javy looked at in the clubhouse. The play occurred in the sixth inning on a shallow pop-up to center field when Baez fell to the ground and into the path of Heyward's legs.
NL MVP Kris Bryant ended a season-opening 0-for-14 slump by just beating out an infield single that bounced off third base in the fifth.
Javier Baez left Friday's game in Milwaukee after the Cubs second baseman collided with teammate Jason Heyward as both players went after a blooper off the bat of Hernan Perez.
Baez gave a thumbs-up from the ground as he waited for the trainer to reach him, but he was clearly shaken up by the play.
Pirates 5, Braves 4: David Freese and Francisco Cervelli hit back-to-back home runs, and Pittsburgh won its home opener in snow flurries over Atlanta.
"It was scary", said Baez, who had some swelling above his bloodshot left eye. "He insisted he was good, but I wasn't convinced".
"Aside from a few pitches here and there, I feel like I did that and hopefully I can take that going forward".
I'm certain he's going to be fine by tomorrow or for tomorrow, but I wasn't feeling strongly at that moment. It's absolutely the right call by the Cubs, especially with the fear of a concussion. Broxton wasn't in the starting lineup but entered the game as a defensive replacement. He has said: "As soon as I turned, I saw J-Hey right in my face".
Cubs: RHP Kyle Hendricks, who led the majors with a 2.13 ERA in 2016, makes his season debut.
OUR PICK: The under has cashed in nine of Hendricks' last 10 starts versus the Brewers and are 4-1 in his last five road starts against Milwaukee.
Heyward pointed to eye black on his forearm and knee, smudges that might have come from the collision with Baez. The veteran left-hander moved into the rotation when Opening Day starter Junior Guerra was lost to a strained calf just three innings into the season opener.