As temperatures climbed into the low 90s, Stadler, of Englewood, Colo., built a three-stroke lead over the field heading into the second round at Point O'Woods Golf and Country Club.
'I got some good breaks out there and played well,'' said the 22-year-old Stadler, a second-team All-American as a senior this year at Southern California. 'I was able to string a whole round together.''
The round matched Stadler's previous tournament best.
Tied for second were 1997 Western Amateur champion Danny Green, 45, of Jackson, Tenn., and Randy Lowry, 15, of Spring, Texas, who is playing in his first Western Amateur.
After a second 18-hole round Thursday, the field of 156 will be cut to the low 50 scores and ties. Thirty-six holes of medal play are set for Friday, with the low 16 for the four rounds advancing to match play Saturday. The semifinals are Sunday morning and the championship match is set for Sunday afternoon.