CHARLOTTE, N.C. -- The third time, Thomas Davis quit.

Out on the field, his Carolina Panthers were battling the Packers in the fourth quarter of the second game of the 2011 season. But underneath the stands, in the team doctor's office, Davis' right knee bulged with fluid and blood. Just as it had both other times.

Just a few minutes earlier, Davis had been trying to tackle the Packers' James Starks when one of his own linemen accidentally leg-whipped him. Now, Dr. Pat Connor did the Lachman test and the pivot-shift test -- the two standard ways to check for a torn anterior cruciate ligament. He moved Davis' knee joint up and down and side to side. It went where it was not supposed to go. Davis and Connor and Panthers trainer Ryan Vermillion talked quietly in the little room. Through the concrete, they could hear the muted boom of the 73,000 fans outside, like a party at the neighbor's house. The diagnosis wouldn't be official until they did an MRI. But Davis knew.

#58 LB

Carolina Panthers

2013 STATS

Tot 117

Solo81

Ast36

FF1

Sack4.0

Int 2

Thanks for everything you did for me, he said. I'm done.

He called teammate Jon Beason, who was in bed recovering from his own surgery for an Achilles tendon tear. This is it, Davis said. No one's going to want me.

That night, he talked it over with his wife, Kelly. This time, he'd get a cadaver ligament to replace his ACL. It wasn't flexible enough for pro football, but maybe he could shoot hoops at the gym. The rehab wouldn't be as hard. He wouldn't have to train for the torque and thrust of life as an NFL linebacker. He'd stop when he was well enough to run around the backyard with his kids.

At first, he felt peace. No NFL player -- no pro athlete of any kind -- had come back from three ACL tears on the same knee. There was no point in thinking about it.

Then he thought about it.

He thought about all he had gone through that everyone knew about. He thought about the one thing almost nobody knew about.

The next morning, he showed up at Panthers practice. And he got ready to start over again.

• • •

Now, two years later:

Against Atlanta on Nov. 3, Davis is lined up as an edge rusher but backpedals into coverage. Matt Ryan throws a deep pass down the middle. Davis leaves his man, sprints for the thicket converging on the ball, and, when it's tipped, he dives for the interception 30 yards downfield.

The next week, against San Francisco, Davis crouches in his outside linebacker spot. The 49ers open a hole in the middle and running back Kendall Hunter cuts into the gap -- until Davis fills it and sticks him so hard the ball flies 5 yards backward. The Panthers recover in a game they go on to win 10-9. "DVR this," announcer Brian Billick says. "Davis with a classic. Look at that. See what you hit, keep your legs churning. That's how you make a tackle at linebacker."

Thomas Davis, 30 years old, on a knee ripped apart three times, is the best he's ever been. He's playing the strongside and the weakside linebacker spots for a defense that has given up the fewest points in the league (221 through 15 games). In the Panthers' first showcase game of the season -- a 24-20 Monday night win over New England -- Davis had 17 tackles. That helped make him NFC Defensive Player of the Month in November. On Sunday, he had 14 tackles and an interception in the Panthers' playoff-clinching 17-13 win over the Saints. And now that Davis is all the way back, he won't leave. The Panthers have played 943 defensive snaps this season, and Davis has been on the field for 919.

To keep his knee together, surgeons have scavenged his body for parts. The first time, they replaced his ACL with part of his hamstring tendon. The second time, they shaved off a piece of the patellar tendon in his right knee. The third time, they needed part of the patellar tendon from his left knee -- so he had surgery on both knees at once.

Now he gets acupuncture on the knees and a balky shoulder. He owns a hyperbaric chamber to force more oxygen into his blood. He deals with the everyday car crashes that make up life in the NFL. The bridge of his nose is perpetually skinned where the rim of his helmet keeps bashing it. Against Miami, he dislocated his right pinkie so badly the bone tore through the skin. He didn't miss a down.

At home, the cape comes off. When he and Kelly decided to get married in 2008, he dug so deep into picking out flowers and color schemes that she started calling him The Bride. She doesn't like shopping, so he gets the groceries and picks out clothes for their four kids. They play cards and board games. The Davises are competitive. Sometimes he and Kelly need a cooling-off period after a rough round of Monopoly.

His injuries put him in an odd place on the field. His opponents worry about him even as they try to whip him on every play. In the San Francisco game, Davis dove on a loose ball and 49ers quarterback Colin Kaepernick landed on Davis, bending his right leg backward. After the whistle, Kaepernick jumped up, worried: "Are you OK? Are you OK?" (Slight knee strain; didn't miss a play.) In the Panthers' 27-6 win against Tampa Bay, while officials sorted out a replay, Bucs offensive tackle Donald Penn wandered over and said: "It's so good to see you out here, man."

Thomas Davis already has registered career highs in sacks (4.0) and tackles (117) this season. AP Photo/Gary Wiepert

It matters to him that he is noticed. It hurt him a little in 2012, his first year back from the third knee surgery, when it became clear Peyton Manning was going to win Comeback Player of the Year. But it matters more to him that he is needed. He missed 39 games over three years (2009-11), but the lost games didn't bother him as much as the lost time in the locker room, pushing his teammates, having them push him.

"There's so much I can't get back," he says. "Now that we're making a run at the playoffs, I have to appreciate every moment. And I have to get my teammates to do the same."

In the New England game, Panthers defensive end Charles Johnson took a shot to the knee and pounded the turf in pain. It turned out to be just a sprain, though, and he went back into the game to help stop the Patriots on their final drive. In the locker room afterward, Davis, a three-time team captain, went over to check on him.

"I was scared," Johnson said. "I heard that s--- pop. I thought I was done."

"But you were all right," Davis said. He walked away, off toward the showers. "We needed you. Big game. Had to have you. And you were there."

He was talking to Johnson. But maybe also to himself.

• • •

The second time, Thomas Davis sobbed in his wife's arms.

He had been doing a simple noncontact drill in a Panthers offseason workout in June 2010. He was supposed to backpedal, turn left, turn right -- a basic move a linebacker would make in a game. When he planted his right leg, his knee gave way. He was done for the season three months before it started.

Davis thought his career could be over in 2011 when he tore the ACL in his right knee for the third time. AP Photo/Gerry Broome

He had always thought he was the toughest man on the field. Growing up in little Shellman, Ga., they played kill the man with the ball, and, when he got the ball, nobody could catch him, much less kill him. On the football team at Randolph-Clay High School, he played eight positions. At the University of Georgia, he shuffled between linebacker and safety and made All-America teams anyway. The Panthers drafted him in the first round in 2005. By 2006, he was a full-time starter. His career had gone just as he had dreamed -- until his knee failed him once, and now again.

One night, he and Kelly were talking about it and he broke down. She held him in her arms and rocked him like a child.

One day not long after, their son Thomas Jr. sat on Davis' lap. Thomas Jr. was 3 then. Davis' leg was bandaged.

Daddy, did you hurt your knee again? Thomas Jr. asked.

Yeah, I hurt my knee again.

Well, Thomas Jr. said, why don't you just stop playing football?

I've never quit anything, Davis told his son. I'm not a quitter.

But he knew it was a good question. He and Kelly are stout in their faith. They believe in signs. They wondered whether his knee giving out over and over was a sign from God that Thomas should quit. They thought about this a lot.

They decided God must be sending some other sign.

• • •

Doubters are the Duraflame logs of every pro athlete. The most gifted and adored player can flip back through the files and find a slight to seize on. Davis remembers that Georgia was his only FBS scholarship offer. He remembers the scouts who said he was too small to play linebacker in the pros. He remembers the comments from each time he hurt his knee. They showed up at the bottom of every online story about him. They trailed him on his Twitter feed. "He could have been real good in the nfl sucks that he can never stay healthy," wrote a commenter on Pro Football Talk. "He should just hang it up though."

Davis' only FBS scholarship offer came from Georgia, where he was a consensus All-American in 2004. A. Messerschmidt/WireImage

"I use that," he says. "I use that as motivation."

Competition motivates him, too. He and Beason, practicing on the backup kickoff team, went all out to see who could get to the goal line first. When Beason tore his Achilles, Davis carried his jersey onto the field the next week -- the very game in which Davis suffered his third ACL tear. Vermillion, the trainer, put them together as rehab partners. They worked out together, 9 a.m. until they gave out, six or seven days a week.

Davis spent weeks just bending and stretching his leg, reclaiming his range of motion one agonizing millimeter at a time. Then he moved to a stationary bike. "I can remember they jacked the seat up so he'd just have to make one little circle," Beason says. "He got up there and tried to pedal and his legs are shaking, he's screaming, that one time from that one circle."

Every ACL rehab equals months of agony. Davis remembers standing up after surgery and the blood flowing into the torn-up spots, the pain so deep it brought tears. Cybex machine, stationary bike, leg extensions, breaking up scar tissue, testing the joint, climbing stairs, learning how to run again. Imagine going through all that. Imagine doing it three times.

Beason made it back, too, but then hurt his knee and shoulder. While he was gone, 2012 first-round pick Luke Kuechly stepped in and became a star. This year, the Panthers traded Beason to the Giants, where he has been a leader on their defense. He and Davis still talk on the phone, reminding each other what they went through.

"Thomas is mentally strong," Beason says. "But I think having the same injury over and over again has to do something to your psyche. To do this for a living, you have to trust your body."

It's one thing when the critics doubt you. It's another when your own body does. And the doubt went into places no one else suspected.

• • •

Davis carried Jon Beason's jersey before a 2011 game. After Davis had his third knee surgery, he and Beason became rehab partners. AP Photo/Mike McCarn

The first time, it stunned him.

It was the eighth game of 2009, Davis' fifth season. The Panthers were playing the Saints at the Superdome. Davis ended up in coverage on Saints receiver Devery Henderson. He planted his leg to bump Henderson off his route. He heard the pop and felt the shock of pain. He was naturally knock-kneed -- which put more stress on the joints -- but he had never been seriously hurt before.

His first thought was Kelly. She was pregnant with their second child together (they have two others from previous relationships). He wanted to do the rehab as fast as he could so he could help out after she had the baby.

But the truth was, he felt lucky to have been playing at all.

This is a story he hasn't told.