MILWAUKEE — Entering his first big league game in 16 months, Joe Nathan felt his heart racing while doing a full sprint to the mound.

Nathan has 377 career saves, and yet Sunday still felt like opening day all over again for the 41-year-old reliever.

Anthony Rizzo's three-run double in the seventh inning helped the Chicago Cubs overcome a four-run deficit and made a winner of Nathan in a 6-5 victory over the Milwaukee Brewers.

"It was opening day for me today, and the adrenaline was ... through the roof. It was almost too much," Nathan said. "I almost needed a defibrillator for the run into the mound."

The six-time All-Star joined a club with World Series aspirations hoping to shore up its bullpen. The heart of the order finally came up with hits after the Cubs struggled with runners in scoring position against Brewers starter Junior Guerra.

That changed after Will Smith took over for Guerra with one out in the seventh.

Tommy La Stella, who went 3 for 3, had an RBI double before Smith (1-3) walked Kris Bryant to load the bases for third-place hitter Rizzo. He hit a 2-2 slider down in the zone into right-centre to clear the bases and give Chicago a 5-4 lead.

Ben Zobrist's RBI single scored Rizzo in the seventh and gave the Cubs a needed insurance run after closer Hector Rondon gave up a homer to Kirk Nieuwenhuis with two outs in the ninth to draw Milwaukee within a run.

Rondon struck out Jake Elmore looking for his 18th save.

"It was a complete victory. Rizzo had a couple tough at-bats but got a really big hit," manager Joe Maddon said.

It completed a successful comeback from Tommy John surgery for Nathan, pitching in his first game in the majors since April 2015. He was activated off the 60-day disabled list earlier Sunday.

Cubs pitchers laboured for much of the afternoon, with starter Jon Lester struggling through four innings, allowing four hits and walking five.

Nathan gave up a leadoff triple to Jonathan Villar in the sixth and a walk before striking out 3-4-5 hitters Ryan Braun, Jonathan Lucroy and Chris Carter.

"Certainly a lot of moments in the game. We had some opportunities to add on runs in general, just to score runs and we didn't capture those opportunities," Milwaukee manager Craig Counsell said.

BRINGING THE HEAT

The Brewers left the retractable roof at Miller Park open on a sticky afternoon, with the game-time temperature at 90 degrees. The contest lasted more than four hours.

"It was without a doubt the hottest game of the year, and it was one of the longer games, so guys were tired, for sure," Counsell said.

ON THE MOUND

Smith's struggles overshadowed another solid start by 31-year-old rookie Guerra, who allowed five hits and one unearned run over 6 1/3 innings.

But the lead evaporated against Smith, normally one of the league's toughest left-handed relievers. Left-handed hitter La Stella and Rizzo both got to Smith for clutch hits.

Smith's ERA ballooned from 1.93 to 3.86 after failing to record an out while giving up five runs on three hits, a walk and an error.

SWIPE CITY

The Brewers built their 4-0 lead, stealing six bases in a game for the first time since May 13, 2013. Braun and Villar each had two steals off Lester by the bottom of the second.

TRAINER'S ROOM

Cubs: Nathan signed with the Cubs as a free agent in May and was on the DL until this weekend. Right-hander Adam Warren was optioned to Triple-A Iowa in a corresponding move. ... SS Addison Russell left the game before the bottom of the fourth with a left heel contusion.

Brewers: Will Middlebrooks left the game before the top of the fourth a lower right leg strain. The third baseman popped out in the bottom of the third.

UP NEXT:

Cubs: Jake Arrieta (12-4) takes the mound when Chicago travels to the South Side for an interleague series against the crosstown-rival White Sox. The right-hander is 3-1 with a 3.82 ERA in six starts against the White Sox.

Brewers: Chase Anderson (4-10) opens a four-game series against his former club, the Arizona Diamondbacks. The righty has struggled over his past six outings, going 0-4 with a 9.13 ERA.