Simpson had five straight birdies in a 67 to match Watson, who has had at least a share of the lead after all three rounds, at 12-under 204 at TPC Louisiana.
Watson needed to birdie the 588-yard 18th to get into the final group Sunday. After hitting his 250-yard approach into a greenside bunker, he blasted out to 4 feet to set up his birdie putt.
"Knowing that if I stroke this well, I'm tied for the lead and not one back, knowing I'm in the final group," said Watson, the Torrey Pines winner in January for his second tour title. "There was a lot of pressure on that for me. Somehow it went in dead center, and so I'm in the final group."
John Rollins (69) was third at 11 under, and 2002 winner K.J. Choi (67) was 10 under along with Steve Stricker (68), George McNeill (65), Charles Howell III (66), Tommy Gainey (68) and Matt Jones (69). David Toms, the 2001 Zurich winner, topped a group at 8 under after a 67. Luke Donald, who missed an opportunity to jump from No. 3 to No. 1 in world rankings a week ago when he lost in a playoff to Brandt Snedeker at Hilton Head, was 7 under after a 70.
Simpson, winless on the PGA Tour, birdied Nos. 3-7, then closed with 11 pars. He has a share of the 54-hole lead for the first time in his career.
"I've always slept pretty good going back to college and amateur days holding the lead," said Simpson, who tied for second behind Phil Mickelson on April 3 in Houston. "But this is why we do what we do. This is why we work out, why we practice, to give ourselves a chance to win on the PGA Tour. I think that will kind of calm my nerves."
Thompson, 16, could be youngest winner
Alexis Thompson has a shot at becoming the youngest winner in LPGA Tour history. The 16-year-old Floridian shot a 5-under 67 to take a share of the third-round lead with Song-Hee Kim in the Avnet LPGA Classic at Mobile, Ala. Kim had a 70 to match Thompson at 7-under 209 on The Crossings course. Thompson wasn't thinking about making LPGA Tour history, just about making shots. "I'm going into [Sunday] just going all-in on every golf shot," Thompson said. -- AP