When that cushion was in jeopardy in the final seconds, the secondary came through one more time for the No. 9 Bears.
Petty threw two touchdown passes and ran for a score, Baylor returned two interceptions for touchdowns and turned away an upset bid with a late pick in the end zone for a 41-38 victory against TCU on Saturday.
"For the defense to have played like they did, I mean, I can't say enough about the defense," Petty said. "The defense is the MVP of this team, the way they turned it around, the way they kept us in games."
The Bears (10-1, 7-1 Big 12) scored 21 consecutive points on either side of halftime with just 1 yard from their high-powered offense and bounced back from a blowout loss at Oklahoma State to maintain their hopes for a share of the conference title.
Baylor could win the league outright and qualify for a BCS bid, likely the Fiesta Bowl, if it beats Texas and the Cowboys lose to Oklahoma next Saturday.
The Horned Frogs (4-8, 2-7) were in position to tie at the Baylor 23 in the final seconds when Casey Pachall decided to go to the end zone and threw his third interception.
Sam Holl dove in front of Brandon Carter just as the ball reached the TCU receiver inside the 5, causing it to pop into the air off Carter's hands and into the arms of Terrell Burt as he crossed the goal line.
Baylor's Orion Stewart and Eddie Lackey had interception returns for scores off Pachall on either side of halftime.
"It came down to my three interceptions that cost us the game," said Pachall, a senior playing his final game after missing most of last season to enter a substance abuse program following a DWI arrest and getting knocked out of five games this year with a broken arm.
TCU's second season in the Big 12 ended with the school's fewest wins since going 1-10 in 1997.
"The bottom line is, we should have beat them," TCU coach Gary Patterson said. "That was our bowl game."
The Baylor offense had running backs Lache Seastrunk and Glasco Martin for the first time after both missed two games with injuries, and the Bears started fast.
Baylor ran 40 plays in the first quarter and had 201 yards before sputtering the rest of the way and finishing with a season-low 370 yards -- 292 yards off their nation-leading average of 662. The Frogs outgained the Bears by 40.
Seastrunk had 92 yards against the Big 12's best rushing defense in the first half, but just 2 after halftime.
"We couldn't ever maintain any continuity," said Baylor coach Art Briles, who was on the sideline three days after his older brother, Eddie, died from an apparent head injury after a fall. "Some of that's got to do with TCU."
Pachall erased some of a 34-17 deficit built on his mistakes with a 4-yard touchdown run and a 16-yard scoring pass to Josh Doctson.
The first scoring drive was kept alive when Baylor safety Ahmad Dixon was penalized for targeting and ejected after hitting Trevone Boykin with his helmet on a third-down incompletion.
Patterson said a coach from the Baylor sideline came across the field to yell at him after Dixon was thrown out. He never specified which coach.
"If that's what class is, then I don't want to be it," said an agitated Patterson, who had his first losing record since 2004 and just his second in 13 seasons at TCU.
The two interceptions returned for scores came in a span of just four passes from Pachall, who was 20 of 45 for 267 yards and two scores.
Stewart jumped an out route and went 82 yards after a slipping tackle from Carter, and Lackey stepped in front of a slant and ran untouched 54 yards to put Baylor up 34-17.
Before the two interception returns, Petty had a 1-yard scoring run after TCU's B.J. Catalon ran into the back of one of his linemen at the 10 and fumbled. Beau Blackshear recovered at the 1.
After Pachall's first scoring pass made it 34-31, Baylor went back up 10 when Petty completed a 26-yard pass to Clay Fuller on third down, then found Levi Norwood on a 33-yard score. Petty had 206 yards passing.
TCU answered when Petty threw just his second interception of the season to Sam Carter, setting up TCU on the Baylor 25. Pachall fooled Stewart with a pump fake for an easy 22-yard score to David Porter on the first play of the fourth quarter to make it 41-38.