CHICAGO -- Chicago Cubs left-hander Jon Lester quickly realized that he did not have great command of his pitches Saturday night.

"I had to battle through it," Lester said. "Really, your mentality there is to keep the team in the game. With this offense, you keep them within reach, and there's always a chance."

Lester's teammates proved him right.

Anthony Rizzo, Kris Bryant and David Ross each hit home runs as the Cubs rallied for a 4-3 victory over the Pittsburgh Pirates. Rizzo finished 2-for-3 with a double, a home run and a walk.

Chicago (46-20) improved to a season-best 26 games over .500. The Cubs are off to the most successful start in franchise history since 1918, when the team started 47-19.

"A well played game on both sides," Cubs manager Joe Maddon said. "Up and down the lineup, everybody contributed. It was a really interesting and well played game on our part."

Jordy Mercer went 2-for-4 with a double, a home run and two RBIs for the Pirates. Pittsburgh (33-35) lost for the ninth time in the past 10 games and fell to 1-7 against the Cubs this season.

Lester (9-3) fought through a rocky first inning to win his fifth straight decision. He gave up three runs on seven hits in six innings. He walked three, all in the first inning, and struck out seven.

Left-hander Jonathon Niese (6-4) drew the loss after surrendering three home runs in 5 1/3 innings. Niese allowed four runs on six hits and four walks. He struck out five.

Pirates manager Clint Hurdle said the home runs spoiled an otherwise positive night for Niese.

"He executed his game plan very well outside of three pitches," Hurdle said. "The 0-2 breaking ball to Rizzo hung over the plate, the fastball to Bryant got in a bad spot, and Ross clipped him with a fastball."

Cubs right-hander Hector Rondon recorded the final four outs for his 12th save.

Ross clubbed a go-ahead home run in the sixth inning to give the Cubs a 4-3 advantage. The 39-year-old climbed the dugout steps for a curtain call after the solo shot, which ended Niese's day.

"My second curtain call ever in my career, and both of them have happened here," Ross said with a smile. "You get to come up and tip your hat. I don't think anybody dislikes that. That's a great feeling, and one that I'll cherish for a long time."

The Pirates grabbed a 1-0 lead on the third pitch of the game when Mercer drove a low fastball into the left-field bleachers. The blast marked Mercer's first career home run to lead off a game.

The Cubs evened the score at 1-1 on a home run by Rizzo in the second inning. Rizzo launched Niese's breaking pitch down the right-field line for his 16th home run.

Pittsburgh regained a 2-1 lead in the third inning. Andrew McCutchen beat the Cubs' shift with a dribbler between first and second base to score Mercer, who led off the inning with a double.

"(Lester) usually works the corners, and I was able to get a couple pitches out over the plate and put good swings on them," Mercer said.

The Pirates increased the lead to 3-1 when Lester hit Mercer with the bases loaded in the fourth inning.

A squeeze bunt by Ross sliced the Cubs' deficit to 3-2 in the fourth inning.

Bryant evened the score at 3 in the fifth inning with a towering home run that cleared the left-field bleachers and landed on Waveland Avenue. The blast traveled an estimated 448 feet.

Cubs center fielder Dexter Fowler exited the game after injuring his right hamstring in the first inning. Fowler pulled up lame while running out a ground ball to third base.

Maddon said initial reports on Fowler were encouraging. Fowler will have an MRI exam on Monday.

"We have to check it out to make sure," Maddon said.

NOTES: Pirates RHP Juan Nicasio was placed on the restricted list Saturday because of personal reasons. Nicasio, 29, is expected to miss his next scheduled start Tuesday against San Francisco. "He needs to get back home," Pirates manager Clint Hurdle said. "We've encouraged him to go." ... Cubs OF/3B Kris Bryant returned to the starting lineup after missing one game because of a stomach ailment. ... The Pirates recalled RHP Curtis Partch and 1B Jason Rogers from Triple-A Indianapolis on Saturday. To clear space, RHP Rob Scahill was optioned to Triple-A Indianapolis and LHP Cory Luebke was designated for assignment. ... Cubs OF Jorge Soler (strained left hamstring) is making progress, but his return date remains uncertain, manager Joe Maddon said. ... The Pirates selected the contract of RHP Jorge Rondon on Saturday.