The two players finished four rounds at 23-under-par 265.
Watson had held the lead but double-bogeyed the last hole for a 69 and fell into a tie with Prugh, who carded a 70.
"I was sluggish all day," Watson said after his round.
Prugh, the third-round leader, is playing in just his third event on the PGA Tour.
Players will go out again Monday at the La Quinta Country Club to play the final round in the tour's only 90-hole tournament.
Rain forced postponement of play Thursday and caused delays Friday.
Tim Clark (66) and Bill Haas (66) were tied for third place at 266 and Joe Ogilvie (68) was in fifth at 267.
Copyright 2010 by United Press International.
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