New world No.1 Rory McIlroy (65), and former No.1s Tiger Woods (68), Luke Donald (69) and Martin Kaymer (70), were in a bunch tied eighth, but eight shots from Watson.
Scott was making no excuses.
"I finished terribly,'' said Scott. "It's just not acceptable.
"I was loose, tired and it was a weak finish.''
"It's just not on.''
Scott's performance was the more surprising given 29 players shot in the 60s and only 15 were over par.
While Watson, Bradley and McIlroy were tearing the course apart, Scott was just holding on at two-under on the day through 15 holes.
"I wasn't worried about the others,'' he said. "I wasn't quite as sharp today and I wasn't charging at all but I was content just plodding because I was getting it around fine and I felt like if I could get it in at 12 or 13 under I'd be in great shape for tomorrow.
"But the last three holes were so costly which is a shame because it has put me out of the tournament completely.''
The 31-year-old Queenslander was still aiming to finish strongly in his final competitive round before next month's Masters.
Greg Norman shot a final round 62 on the same course in 1990 to force his way into a play-off and then won the title on the first extra hole with an eagle but Scott wasn't entertaining thoughts of such miracles.
"I have to just go and have the best score I can and hopefully play like I have been for all but three holes to feel good going forward,'' he said.
"Then whatever happens results-wise I'll be fine.''
Despite holding an impressive lead Watson said he'd be far from comfortable tomorrow.
Already this season on the tour winners have come back from six (Phil Mickelson), seven twice (John Huh and Brandt Snedeker) and eight (Kyle Stanley) shots starting the final round.
"If anybody says they are not nervous going into Sunday that's around the lead or close to the lead or has a chance to win, they are just lying to you,'' Watson said.
"Their psychologist is telling them to lie to themselves.''
Greg Chalmers was the next best Australian, shooting a 68 to move to seven-under and a tie for 18th.
Jason Day (-6), John Senden (-5), Aaron Baddeley (-5), Marcus Fraser (-3) and Geoff Ogilvy (even) all improved their positions from overnight but remain too far back to mount a serious challenge.
-AAP
