Roberts vaulted into third place with an 8-under 62, which set a record for the lowest score ever in a USGA open championship. It also tied the record for the lowest score at any USGA event (Christina Kim, 2001 U.S. Girls' Junior qualifying).
Roberts' bogey-free round included five birdies on the front nine and three on the back, including a 25-foot birdie putt at the 17th hole. He needed just 24 putts to finish the round.
Roberts, 51, has made quite an impression in his first year on the Champions Tour, winning four times since starting out as a part-timer last July. He claimed three of the first four tournaments this year.
'I'm looking forward to being in the chase tomorrow afternoon,' Roberts said.
Watson, meanwhile, will be seeking his fifth major championship and ninth victory overall on the Champions Tour. The 39-time PGA TOUR winner stands at 8-under-par 202 after a round that included seven birdies and three bogeys.
'There's pressure on me, but I've dealt with pressure before,' said Watson, a Kansas City native and honorary Prairie Dunes club member. 'That's the way I deal with it -- I try to push it off to the side, and I do a pretty good job of that.'
So much for Prairie Dunes playing like a U.S. Open course again: 16 players broke par on Saturday after only 11 did so in a full field on Friday.
Defending champion Allen Doyle has been in the 60s all week. He shot a 3-under 67 in the third round and sits three strokes back at 5-under-par 205.
Last year, Doyle came from nine shots behind to win on Sunday. On the putting green Saturday, he noticed players ahead of him were going low in their third rounds.
'You had a feeling that either the USGA screwed up and they made it too easy today, or it was a day that you could score,' Doyle said. 'That's not a bad mind-set when you get to the course. If you see other guys scoring well, then you feel that you can score well.'
Doyle will play in the final group with Watson and Roberts on Sunday. It figures to be a lovefest -- not only between the gallery and home favorite Watson, but between the players as well.
'Hopefully I'll be able to get a chance to play with Mr. Watson,' Roberts said after his round, 'because I absolutely love to play golf with him.'
'It's fun playing with both of them,' Watson said. 'They both have a little bit different approach to the swing.'
'I'm going to have my wife quiz a couple of people to see if they know who else is playing in the group,' Doyle said, commenting on Watson's popularity. 'It ought to be a happening.'
Peter Jacobsen and Mark James share fourth place 4-under-par 206. Jacobsen shot a 68 Saturday, while James dropped from a tied for second place overnight despite a 69.
Andy Bean fired a 64 and is tied for sixth place with Bob Gilder and Morris Hatalsky at 3 under.
Scott Simpson is one shot further back, and Fred Funk, who is making his Champions Tour debut, shares 10th place with three-time 2006 winner Jay Haas and Bruce Lietzke.
Inclement weather delayed the start of round three by 85 minutes Saturday.
More problems are anticipated for Sunday, so tee times were pushed up to the early morning beginning at 8 a.m. local time. The leaders are scheduled to tee off at 9:39 a.m. local time.