Watson, who won last week's Senior British Open in a playoff, began the day four shots back of Stadler, who tied the Senior Open's first-round record with an opening 64. Watson, who lost in a playoff to Don Pooley in the Senior Open three years ago, hit four shots to within a foot of the flag for birdies, but fell back into a tie for the lead with a bogey on his last hole.
Roberts, playing in just his second tournament for the over-50 set, followed an opening 66 with a 67 that included five birdies and a bogey.
Stadler, a former Masters winner with 13 PGA Tour and eight Champions Tour victories to his credit, was even-par through 12 holes, but birdied the par-3 13th and the par-4 18th to pull into the three-way tie for the top spot. He shot a 69 in the second round.
Scores continued to drop as players took advantage of the soft and receptive greens following Wednesday's heavy rain.
Wayne Levi shot a 67 on the heels of an opening 68 and was two strokes back at 7-under 135. Des Smyth, who lost to Watson in the playoff last week, shot a 66 and was tied with Raymond Floyd at 136. Floyd won the last major championship played at NCR Country Club, the 1969 PGA Championship.
Bruce Summerhays (69) and Mark McNulty (67) were next at 137, with Don Pooley shooting a 67 to get to 139. Also at 3-under was seniors rookie Greg Norman, who had yet to begin his second round.
Hale Irwin, the 1998 and 2000 Senior Open winner, followed his opening 71 with a 69 and was at 140. Tom Purtzer, who started the day a shot back of Stadler after a first-round 65, wilted to a 76 that left him at 141.
Among those with later tee times were 1988 and '89 U.S. Open champion Curtis Strange (who had an opening 71), former major-league pitcher Rick Rhoden (71), 1984 and 1995 Masters winner Ben Crenshaw (71), and defending champion Peter Jacobsen (72).
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