Ten Broeck, who spends most of his year caddying for Tim Herron on the main US Tour, fired a 68 on Friday to finish at six-under-par 134, just ahead of Tom Kite.
Former Major winners Bernhard Langer, Corey Pavin and Tom Lehman are among the players in a six-way tie for third place, a further shot back.
Kite had looked set to take a healthy lead into the weekend after reaching nine under par at the half-way point of his second round, but he dropped four shots on the back nine.
Ten Broeck played for on the US Tour in the 1980s, but gave up playing in the 1990s to become a caddie for players including Jesper Parnevik.
He still plays occasionally, however, and even managed to both play and caddie at the same event as Parnevik in 2009. Ten Broeck actually beat his employer, though both players missed the cut.
"Caddying is a lot easier than playing," said the 53-year-old. "That's why you get paid more money to play."
Should he hang on to his lead until Sunday night, Ten Broeck will earn $500,000 - more than double he has ever collected in a season either as a player or a caddie.