PITTSBURGH — The Cubs are threatening to run away with the NL Central.

Ben Zobrist hit a 3-run home run, Anthony Rizzo also connected, and Jon Lester worked out of a pair of jams to lead the Cubs over the second-place Pittsburgh Pirates 6-2 Wednesday for a three-game sweep.

With their seventh win in eight games, the Cubs improved to a major-league-best 20-6. They have won seven straight road games and completed their first sweep in Pittsburgh since Sept. 7-9, 2012. They lead the division by 6 games.

“We pitched really well in this series. We’ve been doing that the whole year,” Cubs manager Joe Maddon said. “And the offense was good again. But the part that permits the offense to get into the flow of the game is the pitching’s been so good. That we are able to stay even or most of the times on top of the other side.

“Offensively we keep having good at-bats over the course of the game and then have a chance to work people down by the end. That’s pretty much been the road map to this point.”

Zobrist’s third-inning homer put the Cubs ahead after center fielder Andrew McCutchen dropped Rizzo’s two-out liner for an error.

Lester (3-1) scattered 8 hits in 5? scoreless innings, struck out five and walked two. The left-hander has allowed 1 run or none in 5 of 6 starts this season.

Pittsburgh loaded the bases in the fourth and put runners on first and second in the fifth, both times with no outs. The Pirates have lost four straight following a six-game winning streak.

“It says a lot about pitch execution because Jonny

got in some jams and was able to pitch out of it, which was really nice to see,” Cubs catcher David Ross said. “He’s done that a couple times here lately.”

Rizzo doubled and scored on Javier Baez’s fifth-inning double and hit his ninth home run of the season in the seventh off Ryan Vogelsong. Rizzo had 3 hits, scored 3 runs and finished the series with 5 doubles and the home run.

Baez also had 3 of the Cubs’ 12 hits and drove in the game’s final run with an RBI forceout in the ninth. Addison Russell added 2 hits, and Kris Bryant extended his hitting streak to 10 games.

Pinch hitter Matt Joyce had a 2-run homer in the seventh against Adam Warren. Three of Joyce’s 4 home runs have come as a pinch hitter.

Juan Nicasio (3-3), who had won his first 3 home starts after being signed as a free agent, allowed 4 runs (1 earned), 8 hits and 3 walks in 4? innings.

Pittsburgh got 2 hits each from McCutchen, David Freese and Josh Harrison. McCutchen had gone 2-for-19 in his previous five games.

“We had our chances but we didn’t capitalize on them,” Harrison said. “It’s disappointing to get swept, but we’re good about getting past things. We came in, took a shower and now we’re moving on.”

• Rizzo is hitting .362 (17-for-47) with 16 RBI in his last 12 games after hitting .163 in his first 14. He also is batting .442 (23-for-52) in his last 13 regular-season games at Pittsburgh with 9 doubles and 9 RBI.

“Part of it is just that he had to have better luck,” Maddon said. “I saw (his batting average on) batted balls in play was just atrociously low, and that just comes back to you.”

• Cubs right-fielder Jason Heyward (sore right wrist) did not start for a third straight game, but Maddon is optimistic he might play tonight when the Cubs open a four-game series against the Washington Nationals at Wrigley Field.

Pirates catcher Elias Diaz underwent surgery Tuesday to debride his right elbow, and the rookie is expected to be out until mid-July.

• Right-hander Kyle Hendricks (1-2, 3.52 ERA) pitching on eight days’ rest tonight when he pitches against Washington right-hander Joe Ross (3-0, 0.79).

After an off-day, Pittsburgh starts a three-game series Friday night at St. Louis with Francisco Liriano (2-1, 3.86) facing Carlos Martinez (4-1, 2.60).

Liriano is 6-2 against the Cardinals in 13 starts.