Jason Giambi hit a grand slam, Alex Rodriguez sent a three-run homer bouncing into Monument Park and the Yankees battered Brian Bannister and the Kansas City Royals 15-6 on yesterday to salvage a split of the season series.

Cody Ransom added a two-run homer and Xavier Nady also went deep for the Yankees, who began the day six games behind Boston for the AL wild-card spot. Rodriguez finished 3-for-3 with five RBIs, and Derek Jeter went 4-for-4 and scored three times.

New York opens a six-game trip Tuesday at Toronto, and will play 22 of its final 38 on the road — all but 10 of those games against teams with winning records.

Mike Mussina (16-7) gave up three runs in the first but settled down after that, retiring his last 14 batters and improving to 6-0 against teams from the AL Central. It was win no. 266 for the right-hander, tying him with Bob Feller and Eppa Rixey for 34th on the career list.

Despite winning at least 15 games 11 times, Mussina has never been a 20-game winner.

Ross Gload hit a two-run homer and Billy Butler had a two-run double for the Royals, who wasted two chances over the weekend to win their first season series against New York since 1999. Instead, they’ve lost nine of 11 after a modest three-game winning streak.

The previously slumping Yankees broke loose and matched the number of runs they scored through the first two games of the series in the first inning alone, sending 10 batters to the plate and quickly assuming control.

A-Rod struck the first big blow, a drive to left that bounded off the concrete beyond the wall and rattled around like a pinball among the stone monuments. His 28th home run tied the game at 3, but a miserable afternoon was only beginning for Bannister.

The right-hander served up a 1-1 pitch to Nady moments later that he drove off the foul pole in right. After a visit to the mound from pitching coach Bob McClure, Robinson Cano singled, Jose Molina hit a run-scoring double and speedy youngster Brett Gardner — who had the winning hit in the 13th inning Saturday — tripled for a 6-3 lead.

Bannister (7-12) didn’t manage another out after Johnny Damon’s inning-ending fly ball.

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METS 4, PIRATES 0 Johan Santana pitched a three-hitter for his fifth career shutout and the streaking Mets got home runs from Brian Schneider and Carlos Beltran in a 4-0 victory yesterday over the Pittsburgh Pirates.

Argenis Reyes added three hits for the Mets, who have won six straight and 10 of 13. New York began the day leading the NL East by two games over Philadelphia.

Santana (11-7) retired his first 10 hitters before Jack Wilson singled and Freddy Sanchez reached on an error in the fourth. Wilson had two hits for the Pirates, who have dropped four straight and seven of nine.

Santana threw 85 of 113 pitches for strikes en route to his eighth career complete game and second this season. His previous shutout was June 19, 2007, when he blanked the Mets for Minnesota.

The left-hander’s third career three-hitter gave the Mets six consecutive road wins for the first time since 2001. The two-time AL Cy Young Award winner struck out seven, including Adam LaRoche three times, and didn’t walk a hitter.

New York went ahead early against Pittsburgh for the third game in a row.