Els took over as world No 1 from Tiger Woods, who held the position for just one week, and is the 10th player to lead since the rankings began in 1986.
Els won his second successive Buick Open by shooting a two-under-par 69 in the final round, holding off a familiar rival, Jeff Maggert.
Els, at 16-under-par 268, finished two strokes ahead of Maggert, whose three-under 68 left him in second place behind the South African for the second year in a row at the Westchester Country Club.
Maggert also pushed Els down the final stretch in the Open at Congressional last week before stumbling at the end and finishing fourth.
"If there are horses for courses, this is probably my golf course," Els said. "Obviously, I love the place. It's a great feeling to win here again."
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