Rookie Jake Knapp made 11 birdies to take the third-round lead in the Mexico Open as Matt Wallace's challenge faded away, while Padraig Harrington was unable to replicate his form from Friday's second round as the three-time major winner carded a one-over 72 to drop back into a tie for 57th on three under.Knapp had seven birdies as he carded a 63 to move to 19-under-par at Vidanta, four clear of fellow PGA Tour newcomer Sami Valimaki of Finland.The American opened with a pair of birdies and added five more in six holes to reach the turn in 28.Wallace had started the day alongside Knapp in a four-way tie for the lead, but the Englishman could only manage a level par 71 to finish the day eight strokes off the pace. LEADERBOARD He was joined at 11-under par by Scotland's Robert MacIntrye, who shot a 65 with four birdies topped off by an eagle at the 18th.Canada's Ben Silverman, Sweden's Henrik Norlander and Chan Kim were on 12-under-par, completing a top five without a win on the PGA Tour.Harrington had managed a five-under 66 on Friday but despite birdies on one, nine and 14, the four bogeys that blemished his round, including three on the back nine, pushed the Dubliner down the leaderboard. Additional reporting: PA