Roberts' tee shot on the par-3 17th plugged deep in a bunker, forcing him to take a drop for an unplayable lie. Roberts two-putted for a double bogey, and Watson had his three-shot lead back.
Still, Roberts likes his chances on Sunday.
"For me to shoot over par and still be in the last group says a lot about how tough the golf course was," Roberts said.
One of the few players to make a major move up the leaderboard was Senior PGA tournament champion Denis Watson, who began play at 3 over for the tournament but had the best round of the day, a 5-under 67 to move to 2 under overall.
"Anytime you have a great day, it's good," Denis Watson said. "I was frustrated and actually drove the ball better the first two days. I hit maybe a slightly errant shot here and there, and I just got raked over the coals."
Denis Watson and the other contenders might not have an easy time catching Tom Watson on Sunday. But Tom Watson has been in this position in the U.S. Senior Open several times before and come away without a victory.
For all his success in major tournaments in his days on the PGA Tour, Watson has never won a U.S. Senior Open. He has had several chances, finishing second in three of the past five senior opens.
Playing with the lead in front of a partisan crowd in his home state of Kansas in the final round of last year's senior open, Watson was beaten by Allen Doyle.
"Maybe this is my time," Watson said.
And this time, Watson might have the wind in his favor, as the forecast on Sunday calls for 25 mph winds.
High winds are supposed to be the signature of Whistling Straits, and they blew hard during practice rounds earlier this week. But the tournament's first two rounds were played under calm conditions, leading to unexpectedly low scores.
Now, players are happy to make par in some spots.
"When you put the ball on the tee, you're shaking (nervously) because you never know what happens," said first-day leader Eduardo Romero, who shot a 1-under 71 to move to 2 under for the tournament. "You can make seven or eight or three, you never know."
Divots: One day after recording a triple bogey at No. 18, Romero made bogey at the course's tough finishing hole after having to hit a short wedge shot from next to the grandstand into a bunker. ... Watson was left wondering about the accuracy of the "computer" in his head after hitting four approach shots over the green. "Something's wrong with the computer there," Watson said. ... Six players are tied at 2 under.