The left-hander pitched a five-hitter, leading the Cubs to a 2-1 victory over the Pittsburgh Pirates on Tuesday night and a split of a day-night doubleheader between National League contenders.
Lester (10-10) pitched his first complete game for the Cubs, who signed him to a six-year, $155 million contract as a free agent in December, and the 12th of his 10-year career.
"This is what he does," Cubs manager Joe Maddon said. "He likes pitching in big games in the latter part of the season. It's not a surprise."
Lester struck out nine and walked one.
"He's an ace for a reason," Pirates first baseman Michael Morse said. "Guys say they saw him well, but he was able to keep it off the barrel."
The Cubs (83-61) ended a three-game losing streak and got back within four games of the Pirates, who lead the NL wild-card standings. The Cardinals (89-54), who were playing at Milwaukee on Tuesday night, led the Pirates (87-57) by 2 1/2 games and the Cubs by 6 1/2 games in the NL Central pending the result of St. Louis' game.
Left fielder Starling Marte's sacrifice fly in the eighth inning gave the Pirates a 5-4 victory in the first game.
While Lester is excited to be pitching meaningful games in September for a franchise that hasn't been to the postseason since 2008, he is well aware that the Cubs haven't won a World Series since 1908.
"When I signed here, I envisioned winning a World Series, not just playing September baseball," Lester said. "Hopefully, we can get to that point."
Catcher David Ross and third baseman Javier Baez had two hits each for the Cubs in the nightcap.
Pirates left-hander J.A. Happ (5-2) saw his five-start winning streak end as he allowed two runs and eight hits in 5 2/3 innings while striking out eight and walking two.
The Cubs scored a run in the first inning as left fielder Austin Jackson doubled and came home on first baseman Anthony Rizzo's single.
Ross doubled and scored on center fielder Dexter Fowler's double in the second to increase the Cubs' lead to 2-0.
"I got myself in a couple of tough situations and had to really give it everything to try and get out of there," Happ said.
Third baseman Aramis Ramirez had two hits and scored the lone run for the Pirates, who lost for the just the second time in eight games. He came home on Morse's double-play grounder in the seventh inning to get the Pirates within 2-1.
Lester, though, retired his final seven batters in a 111-pitch effort.
"The thing that I loved was the maintenance of stuff," Maddon said. "The fastball still had good carry in the last inning. He was throwing 94 (mph) in the eighth. You could see by the hitters' swings. They were not very comfortable."
In the first game, with the score tied at 4, first baseman Pedro Alvarez drew a leadoff walk from reliever Justin Grimm (3-5) to start the decisive eighth-inning rally. Pedro Florimon ran for Alvarez, stole second and continued to third on catcher Miguel Montero's throwing error before scoring on Marte's line drive to left field.
Marte, the Pirates' regular left fielder, entered the game in the seventh as a defensive replacement with Pittsburgh leading 4-1. He missed a third consecutive start while recovering from stomach flu but was in the lineup for the second game.
Left-hander Tony Watson (4-1) pitched a perfect eighth for the win, and closer Mark Melancon worked a 1-2-3 ninth to tie the franchise record with his 46th save in 48 opportunities. Mike Williams set the mark in 2002.
The Cubs tied the score at 4 during a three-run seventh when they chased Pirates right-hander Gerrit Cole, who was unable to notch his 17th win. Cole gave up four runs (three earned) and six hits in 6 1/3 innings with eight strikeouts and no walks.
Cubs right-hander Jason Hammel failed in his bid to win consecutive games for the first time since June 1 and June 6, getting tagged for four runs and five hits in 3 2/3 innings.
NOTES: The Pirates selected the contract of RHP Vance Worley from Triple-A Indianapolis to add bullpen depth, and they designated RHP Radhames Liz for assignment. Worley was 3-1 with a 2.38 ERA in five starts for Indianapolis after being designated for assignment earlier in the season. ... Cubs 3B Kris Bryant's seven-game hitting streak ended as he went a combined 0-for-7 with five strikeouts in the doubleheader. ... Chicago RHP Jake Arrieta (19-6, 1.99 ERA) will face Pittsburgh RHP A.J. Burnett (8-5, 3.14 ERA) on Wednesday night in the third game of the four-game series. Arrieta has won eight straight starts and will try to become the Cubs' first 20-game winner since Jon Lieber went 20-6 in 2001.