Twice last night, Pittsburgh Pirates rookie left-hander Zach Duke faced showdown situations against Mets slugger Cliff Floyd with runners on base. Both times, Duke reared back and threw third-strike curveballs that measured all of 75 mph.

Those confrontations highlighted Duke’s seven innings of two-hit ball as he won his sixth straight decision, beating the Mets 5-0.

Why the curves instead of going right after Floyd?

“He’s really a fastball hitter,” Duke said, showing he had done his homework.

Floyd was suitably frustrated.

“You can’t miss those, especially with someone who has good stuff,” the Mets’ cleanup hitter said. “It’s such a good one.”

The Mets didn’t have many other chances against Duke, who retired the first 11 batters, did not give up a hit until Chris Woodward’s double in the fifth, and allowed four baserunners. He threw 103 pitches, 69 for strikes.

It was a continuation of an impressive success story for the 22-year-old, who was promoted from Triple-A Indianapolis on July 1 and has been Pittsburgh’s best pitcher for two months. He made it look easy.

“It’s hardly easy, that’s for sure,” Duke said. “It’s come very quickly for me and that’s a little bit of a surprise.”

Duke doesn’t overpower hitters, something Floyd learned. Instead, he changes speeds, mixes his pitches and keeps hitters off balance. His six straight wins is the longest streak by a Pirates rookie since Don Robinson won seven straight decisions in 1978.

“He did a nice job,” Pirates manager Lloyd McLendon said. “He had the sinker and changeup working. His breaking ball was sharp. He continues to show the ability to throw quality pitches when he has to.”

Duke (6-0) struck out five and walked one. The Mets’ first baserunner came with two outs in the fourth, when right fielder Nate McLouth dropped Carlos Beltran’s routine fly ball. The mistake hardly disturbed Duke, who struck out Floyd to end the inning.

By then, the Pirates had a 4-0 lead on two RBI doubles by Jose Castillo and a two-run homer by Brad Eldred against Victor Zambrano (6-10).

Beltran singled in the seventh for the Mets’ other hit off Duke. Duke then struck out Floyd with another 75 mph curve. David Wright walked but Woodward hit into an inning-ending double play. That finished Duke’s night, which included his first major league hit, a seventh-inning single.

“I felt great,” he said. “I controlled the fastball very well. I threw the changeup for strikes when I needed to.”

After he was called up, Duke won three games and posted an 0.87 ERA in his first month in the majors. Last night’s win was his third in August and followed his first poor start, when he lasted just four innings and allowed four runs and eight hits at Houston.

Ryan Doumit singled with two outs in the second and scored on Castillo’s double.

In the fourth, Zambrano hit Jason Bay with a pitch and Eldred followed with his fourth homer of the season, sending an 0-2 pitch an estimated 400 feet into the left-field picnic area.

Doumit then doubled, took third on a wild pitch and scored on Castillo’s second double. He was thrown out at third trying to stretch both doubles into triples.

In the eighth, Pittsburgh bunched singles by Bay, Rob Mackowiak and Doumit for another run.