Jon LesterThe term “ace” is about as subjective as it gets.

If an ace is a perennial All-Star who gives you 20 wins and a sub-2.00 ERA, then no, Jon Lester is not an ace. But if an ace is defined by confidence, composure and the ability to succeed when the lights shine brightest, then Lester is the gold standard.

Lester, who already added to his growing Red Sox legacy with a masterful performance in Game 1 of the World Series, tossed another gem in Game 5 to pull Boston to within one win of a title. The left-hander gave up just one run on four hits in 7 2­/3 innings Monday before handing the ball over to Koji Uehara, who closed out the Red Sox’ 3-1 victory.

“He’s an ace. That’s how aces do it,” David Ortiz said after Game 5. “Lester, he came into the big leagues early in his career, especially when we were going to the playoffs and we were going through these types of games, and he learned, he learned. Every time we have these tough games where we’ve got to come through, he’s one of them. I was super proud in watching him pitch out there and keep it cool, keep it cool and do what he did tonight.”

Lester’s only blemish in Game 5 came when Matt Holliday drilled a solo home run in the fourth inning to tie the game at one apiece. Lester rebounded to retire 13 of the next 14 batters he faced and exited in the eighth inning with a 3-1 lead after David Ross and Jacoby Ellsbury knocked in a pair of runs in the seventh inning.

Lester struck out seven Cardinals hitters in Game 5, and more importantly, didn’t walk anyone. He was extremely efficient, and his pitch count remained under control until the very end. The lefty threw just 91 pitches despite working into the eighth, and the game never sped up on him at any point. It was an exceptional display of pitching from a guy who has come up aces for Boston in the postseason.

“Well, he was pretty good in Game 4 [of the World Series] back in 2007,” John Farrell said when asked if Monday marked as big a game as Lester has ever pitched. “But given the stage, given how strong he’s been throughout the course of this year, and particularly the second half and what he’s doing in his own right career‑wise in the postseason, yeah, this was a big game. … We talked before the game, we felt this was going to be a classic pitcher’s duel. It was shaping up that way. Fortunately, we were able to break through in the seventh inning. Jon Lester was fantastic tonight.”

Lester has now defeated Cardinals ace Adam Wainwright twice in the series while giving up just one run in 15 1/3 innings. When you add in the 5 2/3 shutout innings that Lester provided in Game 4 of the 2007 World Series, the 29-year-old has a sparkling 0.43 ERA (one earned run in 21 innings) in three career starts in the Fall Classic.

Lester is 4-1 with a 1.56 ERA in five starts this postseason — with four of those starts coming against two of the game’s best offenses in Detroit and St. Louis. The effort lowers Lester’s career playoff ERA to 2.11 (18 earned runs in 76 2/3 innings) in 13 games (11 starts).

“He’s been on a great run, particularly this entire second half,” Farrell said. “And he’s maintained his strength. That’s probably the biggest thing. He’s done such a great job at his work in between starts to maintain the durability and strength overall. And it was evident here tonight.”

Lester’s Game 5 performance slaps a bow on a terrific season for the veteran. Despite some midseason struggles, Lester stepped up when it mattered most and proved that when push comes to shove, he’s capable of rising to the occasion and overcoming the other team’s best pitcher.

Ace or not, that’s a card that most teams would kill to have in their deck.

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