Summerhays ended at 12-under 201 after 54 holes, two shots better than Chris Stroud, who fired a 3-under 68 Saturday at El Camaleon Golf Course in Mexico.
Second-round co-leader Will Claxton fell into a share of third place after registering a 1-under 71, ending the day in a tie with Michael Allen (66).
Knotted for fifth were Robert Allenby (70) and Marc Turnesa (67).
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