Randy Johnson came out because he stiffened up a bit – or maybe he was a little tired. Depends who you listen to. One thing is certain, though: the Yankees had fun facing Kansas City pitching.

Johnson won de spite lasting only five innings, and New York got home runs from Gary Sheffield, Jason Giambi,and Johnny Damon to polish off a three-game sweep of the Royals with a 9-3 victory yesterday.

“We all knew it was only a matter of time before the offense started to come around,” Giambi said. “I think guys finally started to relax a little bit. Now we’re coming up with that big hit, driving in that big run.”

The Yankees built a 4-0 lead for Johnson, pulled after only 87 pitches. Manager Joe Torre said the 42-year-old lefty got a little stiff, but didn’t think there was any “structural” problem.

“I think this is just a freak thing,” Torre said.

Johnson (2-1) first said he simply tired a bit,then agreed the trouble could be called stiffness in his right shoulder. Pitching coach Ron Guidry described it as just “general tiredness.”

“Everything’s fine. Everything’s good,” said Johnson, who told Guidry he was done after the top of the fifth.”I don’t need to go out there every time and pitch seven, eight innings.”

He and Torre both said they expect Johnson to make his next start. General manager Brian Cashman also said he wasn’t concerned.

On the mound, the Big Unit looked sharp. He has 16 strikeouts and no walks in three starts. He had never gone two starts into a season without walking a batter.

Bernie Williams, who finished with three hits, and Robinson Cano each had an RBI single off Denny Bautista (0-1) for the Yankees, who have won 14 straight home games against Kansas City – a streak dating to August 2002.

“This is a good time to get out of here,” manager Buddy Bell said.

Featuring a biting slider, Johnson cruised into the fifth with a four-run lead. He had allowed only one hit and faced one more than the minimum.

But with two outs, the Royals strung together three hits. Shane Costa poked an 0-2 pitch to center for an RBI single.

Johnson retired Tony Graffanino to end the inning and didn’t come out for the sixth. Scott Proctor worked 1 1/3 scoreless innings and Mike Myers fanned Costa with two on to end the seventh.

Graffanino homered against Tanyon Sturtze leading off the eighth, cutting it to 4-2. Alex Rodriguez doubled leading off the bottom half, and Giambi’s two-run shot to center off Jimmy Gobble made it 6-2.

Damon added a three-run drive into the right-field upper deck, his first homer with the Yankees, and came out for a curtain call for the crowd of 54,381. He had four RBI.

Every Yankee starter had a hit for the second consecutive day. The Yankees have scored 40 runs in winning four straight overall.The Royals, meanwhile, dropped their fourth straight.

Johnson pitched to Kelly Stinnett, who used to catch the left-hander in Arizona as well. Torre said before the game he might stick with that combination if it works well.

John Flaherty was Johnson’s personal catcher last season, and Torre acknowledged that Johnson and regular starter Jorge Posada didn’t work well together – perhaps because of their different personalities, the manager said.

But Torre and Posada said yesterday there was no issue or problem between the star pitcher and catcher.

Sheffield began to refocus Wednesday on using the entire field, and he did a good job of that again in the first inning yesterday with a home run to right-center. It was his second homer in two days and the 452nd of his career, tying Carl Yastrzemski for 27th on the all-time list.

The Yankees opened the second with four straight singles and pushed the lead to 4-0. Williams had his 1,200th RBI, seventh on the club’s career list behind six Hall of Famers.