WASHINGTON -- Nationals left-hander John Lannan and catcher Wil Nieves make a great tandem. Ever since manager Manny Acta paired them against the Mets on April 17 at Shea Stadium, Lannan has been virtually unstoppable.

With Nieves behind the plate, Lannan has given up one run in his past 20 innings and hasn't allowed a run in his past 19.

On Sunday, Lannan showed once again that he is the Washington's best pitcher. He threw seven shutout innings and helped the Nationals blank the Cubs, 2-0, at Nationals Park.

"[Lannan and Nieves] have something going right now," Acta said. "So I don't think I'm going to break it up right now."

Washington took two of three games in the series. It's the first time the Nationals won a series since March 31-April 2, when they took two of three from the Phillies.

One cannot say that Lannan, 23, was dominating in the game on Sunday, but he showed how he can get out of jams. He even had a brain cramp during the game.

With one out and runners on second and third in the fifth inning, Cubs center fielder Reed Johnson hit a hard shot at Nationals first baseman Nick Johnson. The ball bounced off Nick Johnson's body, and he had no chance at getting Reed Johnson at first. The only shot Washington had of getting Johnson was if Lannan had covered first base.

"I had a mental lapse when I didn't cover first base," Lannan said. "I didn't know what was going on in my head."

But the young left-hander got out of the jam when Ryan Theriot hit into a double play to end the threat.

In the sixth, the Cubs had runners on second and third with one out. However, Mark DeRosa hit a comebacker to Lannan and Ronny Cedeno popped up to Johnson at first base.

"'You have to forget what happened in the past,' that's what [Acta] told me," Lannan said. "The past is the past for a reason. You got to work on what you got going on right now."

Said Acta of Lannan getting out of jams: "He showed me a lot by being able to make pitches when he had to. That shows a lot for a kid that age."

Lannan said he would not have been successful in his last three starts without Nieves, who is animated behind the plate and keeps Lannan into the game.

"He keeps me on my toes," Lannan said. "We are on the same page. I like the way he calls a game. He has just been great."

It was Nieves who told Lannan to start throwing his back-door slider more often, and the pitch has been deadly to opposing hitters. His best game using the pitch was on April 17, when he struck out 11 batters against the Mets.

While they have only played three games together, Nieves feels that he and Lannan have worked together for years. Working well with pitchers is no surprise per Nieves' track record. He had a good rapport, for example, with Yankees right-hander Mike Mussina.

"[Lannan] doesn't shake me off that much. So it just feels pretty good. I feel like I have been catching him a long time," Nieves said. "I'm real hyper behind the plate. I just want the pitcher to be hyper, too -- to be pumped. That's just me. I have been doing this for 13 years. That's the way I catch. ... I want the pitcher to be hyper so our infielders don't fall asleep."

With catcher Paul Lo Duca expected to come off the disabled list, there is a possibility the duo of Lannan and Nieves could be broken up. Nieves could be back in the Minors by the end of the week. But general manager Jim Bowden sounded like a man who wants to keep the duo together.

"I hate to send anybody down that is performing at this level. That will be a difficult decision when we get there," Bowden said. "Right now, [Nieves] has been a very important part of our winning. I look forward to him catching his next game."

Cubs left-hander Ted Lilly also had a strong outing, giving up two runs on four hits. However, his downfall came in the second inning. Wily Mo Pena singled to left to plate Johnson, while Nieves drove in Pena with a single to center field.

Nieves is now 8-for-16 (.500) in his last five games. Not a bad stretch for a guy who had a .161 career batting average entering the 2008 season.

"I'm having fun hitting," Nieves said. "I know I can do it. I'm trying to stay within myself -- hitting the ball up the middle and the other way. Now, I'm just having fun."

Nieves is also having fun catching Lannan.