Schenk fired a four under 67 on Saturday and Thornberry a 69 to both be 12-under on 201 with hardly any margin for error going into the final round.
Schenk and Thornberry are among 10 players from the top 11 on the leaderboard who arrived in Bermuda outside the top 100 in the FedEx Cup with time running out.
The RSM Classic at Sea Island next week ends the season, the first one that offers cards to the top 100 instead of the top 125.
Schenk is at No. 134 and is running out of options. But he began making minor adjustments in his game the last few months and watched it pay off in a timely way at Port Royal.
Winless in 242 starts on the PGA Tour, he suddenly has a chance he didn't see coming.
"A great opportunity to have for tomorrow," said Schenk, who has a share of the 54-hole lead for the first time since Colonial two years ago.
"I know if I don't, I have to go to Q-school if I don't get inside the top 100, so that's a pretty big motivator."
Thornberry is at No. 178 and only a win would secure a card, coming with a two-year exemption.
But they have plenty of company.
The group one behind includes Adam Hadwin of Canada, who had the 36-hole lead, who had to settle for a 71 to be tied for third with Max McGreevy (69), Chandler Phillips (70) and Takumi Kanaya of Japan (66).
Rikuya Hoshino had a 67 and was two behind.
Adding to the pressure of trying to keep a PGA Tour card was the wind that didn't let up.
Thornberry felt the trick was to take advantage when it helped and hold on for dear life when it didn't.
"Just try to battle it out, that's really just the key to it all," he said.
Schenk was an example in 2023 of players who could work their way into big events even if they didn't get in the $20 million signature tournaments at the start.
He made it to the Tour Championship without winning, getting into all the majors in 2024.
Now it feels like he's starting over, but the 33-year-old from Indiana is feeling good about some of the changes he made in September.
"I wish I really could have done this earlier in the season because I wouldn't be in this position, but here we are," Schenk said.
"And if I can make some putts tomorrow — it's hard to do in the wind — maybe play another bogey-free round, that'd be awesome."
