The American started the day with a one-shot lead and looked on course to remain at the head of the field going into Sunday???s final round after posting four birdies in a flawless first 14 holes at the Palmer Private at PGA West to move two clear on 19 under.
However, he bogeyed the 15th and then the par-three 17th after his tee shot bounced off the rocks, across the green and into the water channel on the left.
Kuchar dropped a further shot on the 18th after pulling his second shot into the rocks on the left of the green and had to settle for a one-under 71 which left him 16 under for the tournament.
As a result he is one shot behind a quartet of Americans with a tight finish in prospect on the Palmer Private lay-out on Sunday.
Bill Haas (69), Erik Compton (67), Justin Thomas (68) and Michael Putnam (69) all share the lead on 17 under with Haas the only previous winner on the PGA Tour.
Kuchar is joined on 16 under by Ryan Palmer, who failed to repeat his Friday 61 fireworks as he carded a 68, Scott Pinckney (69) and Steve Wheatcroft (68).
There is then a two-shot gap back to Scotland???s Martin Laird (68), Germany???s Alex Cejka (70), South African Rory Sabbatini (63), Italian Francesco Molinari (67), Sung Joon Park of South Korea (67),?? American Nicky Watney (71) and his compatriot and defending champion Patrick Reed (67).
Phil Mickelson is six shots off the pace on 11 under following a 68, but English duo Paul Casey and Luke Donald missed the cut along with the new slimline Jason Dufner.