"I was just trying to hit a fly ball to the outfield," Santiago said. "I was trying for one and I got four."
Santiago homered to left on Benitez' first pitch, his fifth homer of the season and seventh career slam the first since Aug. 11, 1997, at Detroit.
"I got a good, solid swing on the ball," said Santiago, who went 2-for-5 with five RBIs.
Reds manager Jack McKeon said he never considered using a left-handed pinch hitter for Santiago after New York brought in the right-handed Benitez.
"If you look down our roster, we didn't have much left, and I would rather stay with Benny," McKeon said. "All good relievers have bad days. He made a mistake and got one up, and Benny took advantage."
Benitez blew a save for the fifth time in 42 chances this year.
"When I gave him the ball I said, 'Pick me up,' " Franco said. "The guy's been automatic all year. But he's a human being, and we all make mistakes."
John Riedling (2-1), the fifth of six Cincinnati pitchers, allowed one hit and walked one in one inning, and Danny Graves pitched a perfect ninth for his 25th save in 30 chances.
Mets starter Bobby J. Jones allowed five runs and six hits in two innings, and Bobby M. Jones followed with four innings of shutout relief.
"I didn't have good stuff at all today," Bobby J. Jones said. "There are times during the season when you don't; there are just those kinds of days."
It was the first time in Mets history that pitchers with the same first and last names followed each other, although Bob L. Miller and Bob G. Miller pitched in the same game five times in 1962.
Reds starter Rob Bell, who pitched a career-high eight innings Friday against Montreal, lasted just 41 pitches and 1 1-3 innings. He gave up five runs and two hits in 1 1-3 innings.
New York, which outhit the Reds 13-6, went ahead in the first on Matt Franco's three-run homer, his first home run since July 16, 1999, at Tampa Bay.
Griffey's home run pulled the Reds within a ruin the bottom half, but the Mets took a 5-2 lead in the second on Pratt's homer and Lenny Harris' RBI single.
Santiago hit an RBI double in the bottom half and Casey tied it 5-all with a two-run homer in the third.
New York took a 7-4 lead in the fourth against Keith Glauber on RBI grounder's by Mike Bordick and Derek Bell, who reached on an error by shortstop Chris Sexton. Mike Piazza hit into a run-scoring double play in the seventh.
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