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TCU long has been more than a mere annoyance to Oklahoma.

TCU won in shocking fashion in Norman in 2005, one of only two home losses to nonconference foes the Sooners have suffered during coach Bob Stoops' 16 seasons at Oklahoma. Last season, the visiting Horned Frogs pushed Oklahoma to the limit before the Sooners won 20-17.

A year earlier, in the first Big 12 Conference meeting between the teams, a fourth-down TCU pass fell incomplete at the goal line, allowing visiting Oklahoma to escape with a 24-17 win. So when No. 4 Oklahoma (4-0, 1-0 Big 12) plays at No. 25 TCU (3-0, 0-0) on Saturday, a close, hard-fought game would seem to be a probability.

On Monday, Stoops allowed that TCU "probably" was the best team the Sooners have played so far this season.

"West Virginia, I thought, was awfully good, too," Stoops said, mentioning the squad the Sooners beat 45-33 in their Big 12 opener. "But TCU looks really, really good and has played really, really well to this point."

The Horned Frogs' calling card under longtime coach Gary Patterson has been their defense and they lead the Big 12 so far this season, allowing an average of 218.7 yards per game. Through the years, TCU's defensive schemes have proven a challenge for Stoops and his staff to overcome. Asked why, Stoops kept coming back to the same word, using a version of "fundamental" four times.

"They're sound fundamentally, really disciplined and they know what they're doing," Stoops said, noting how Patterson has "a set system . that their players get trained in at a young age. They're really fundamental in how they do it."

TCU struggled at times offensively last season, but veteran quarterback Trevone Boykin has looked impressive in 2014 running a system that Oklahoma defensive tackle Chuka Ndulue and Stoops compared to the high-tempo offense used by Texas Tech.

"We've got to be ready for the tempo and him throwing the ball about 80 times a game or something like that," Ndulue said.

TCU is third in the Big 12 in total offense at 532 yards per game and are coming off a 56-0 romp over SMU.

"The biggest difference offensively you see them spreading the ball out now," Stoops said. "Boykin is throwing the ball well and running it. They're using a bunch of players. You see them spread out and they're making you defend the whole field, which that offense does, and defend the whole field and every player. They've done a really good job to this point being highly productive offensively."

Oklahoma will enter the game off a bye week, the first of three this season for the Sooners. Cornerback Zack Sanchez - working on a streak of five straight games with an interception - said his banged-up shoulder feels considerably better after the time off, but Stoops said it's likely running back Keith Ford will miss a second straight game as he heals from a slight fracture of his right fibula, suffered in a win over Tennessee on Sept. 13.

Ford's absence could mean another star turn for freshman running back Samaje Perine, who rumbled for 242 yards in the win at West Virginia. Stoops wouldn't say if Perine - who leads the Big 12 in rushing - earned more playing time with that performance.

"Every game is a little bit different," Stoops said. "We'll see how it goes. You get into the game, certain plays or certain players and what you're doing is working (so) you go to it more. We'll wait and see."

Oklahoma is second in the league in scoring offense (44.8 points per game) and TCU is third (44.7), but Sanchez said he expects another defensive slugfest on Saturday.

"They always play us tough," Sanchez said. "The past two years, they always find a way to play us tough. It's always a defensive battle. We've kind of got to outdo their defense. Those guys want to do the same to us. It's a defensive battle. We've just got to play our game and I think we'll be all right."