No longer a regular starter, Bernie Williams keeps coming up with key hits.

After Pedro Martinez dominated for seven innings, Williams capped a three run rally in the eighth with a go-ahead double yesterday that sent the Yankees to a 5-3 victory over the Mets in the finale of the season’s first Subway Series.

“Obviously, it means a lot,” said Williams, the Yankees’ regular center fielder from 1993 until he lost his job in early May. “I’m just trying to make the best out of the opportunity that I’ve been given. Hopefully, a lot more will come.”

Following eighth-inning errors by third baseman David Wright and shortstop Jose Reyes, Hideki Matsui tied the score for the Yankees with a two-run, two-out single off Roberto Hernandez. Williams then pulled his double down the right-field line.

“It’s taken a little time for him to adjust emotionally,” Yankees manager Joe Torre said. “Today was big for him.”

Williams, starting because of the injury to Sheffield, is 36 and his skills have declined, prompting the Yankees to start Matsui in center.

“As long as we’re winning, it makes it a little easier,” Williams said of his part-time role, adding that his goal was to “work myself into the lineup by hitting, by producing.”

The Yankees, whose starting lineup was missing Derek Jeter (swollen elbow), Gary Sheffield (sore hand), and Jorge Posada (sore shoulder), took two of three at Shea Stadium. Alex Rodriguez called it the biggest regular season win in his 1 1/2 seasons with the Yankees, who had been 0-for-18 this year when trailing after seven innings.

The Mets, without Carlos Beltran (strained quadriceps) and Kaz Matsui (strained upper back), had been 17-2 when leading after seven.

“We could have swept this series,” Hernandez said.

Martinez, 11-12 lifetime against the Yankees, allowed just one run and four hits in seven innings, struck out six, and walked one. Pushed back two days after receiving a cortisone shot to his hip, he was facing his longtime nemesis for the first time in a Mets uniform.

“It was no different at all, except that this time around I had more fans behind me and … less hate words,” he said. “I have nothing to say against the Yankees or anyone there.”

Rodriguez’s seventh error of the season led to a pair of unearned runs in the second against Carl Pavano (4-2). Cliff Floyd’s 12th homer made it 3-0 in the third.

Martinez got out of a bases-loaded jam in a 31-pitch first inning, retiring Jason Giambi on a foul out to Wright, who dove into the stands. That was the first of 13 straight outs for the three time Cy Young Award winner, who needed just 44 pitches to get through the next four innings.

Before 55,953, the largest regular-season crowd at Shea since 1965, Tony Womack started the comeback when he singled leading off the sixth, stole second, and scored on Rodriguez’s single.

Womack reached in the eighth against Dae-Sung Koo when Wright dropped his one-out grounder, and Ruben Sierra followed with a potential double-play grounder to second. Miguel Cairo tossed the ball to Reyes, who dropped it as he came across the bag.

“I tried to be too quick,” Reyes said.

Hernandez (2-2) relieved and Jeter, who was hit by a pitch Saturday, ran for Sierra. The Yankees then pulled off a double steal as Wright failed to cover third, playing back with Rodriguez up.

“We might have crossed up on some signs,” Wright said.

Rodriguez fouled out to first, bringing up Matsui. Mets manager Willie Randolph elected not to intentionally walk Matsui in order to pitch to Williams, in an 11-for-61 slide. Matsui fouled off three straight pitches to the left side before singling.

“The pitch I threw to Matsui was probably the best pitch I threw in the entire sequence,” Hernandez said. “They usually ground to shortstop or pop up that pitch. He stayed with it.”

Pavano, sent to Montreal in the 1997 trade that brought Martinez to the Red Sox, won his fourth straight decision. He allowed three runs, but two were unearned due to Rodriguez’s error.

There were runners at second and third when Martinez, who entered 0-for-21 this year at the plate, hit a bouncer that kicked off Rodriguez’s glove. Reyes followed with an RBI single.

“That was a ridiculous error,” Rodriguez said. “Probably the easiest play that I had all year.”

Mike Stanton, Tom Gordon, and Mariano Rivera followed Pavano, with Rivera getting his ninth save in 11 chances. Womack made it easier for his closer with a run-scoring single in the ninth off Mike DeJean.

“Good teams don’t give other teams second chances,” Hernandez said. “We’ve got to learn we can’t do that.”