No. 1 Clemson's best defense at times has been its offense

No one pushes No. 1 Clemson's defense harder than its offense.

The combination has led to undefeated season, an Atlantic Coast Conference championship and a spot in the national semifinals against No. 4 Oklahoma (11-1) at the Orange Bowl on Thursday.

Sometimes, though, it's hard to determine if Clemson's best defense is its explosive offense, putting up big numbers that none of the Tigers' 13 opponents this season have been able to outscore.

The formula was tested the last two games and almost cost Clemson its undefeated season. The Tigers led rival South Carolina 28-10 before a late Gamecocks rally cut it five, 37-32. in the ACC title game, Clemson was up 35-16 against North Carolina before needing a final second onside kick recovery to hold off the Tar Heels, 45-38.

Containing Oklahoma will be a challenge for Clemson's defense, but maybe not its toughest of the year.

Before the teams arrived in Florida on Saturday, Tigers receiver Deon Cain said in some ways, keeping pace with Clemson's offense is an even bigger motivation.

''If we keep scoring points, that's going to tell (the defense) they need to do better,'' Cain said, ''because they want to be up there competing with us, too.''

But the lapses by Clemson's defense, ranked seventh in the country, raises questions whether it's a national championship caliber unit.

''I don't know,'' Tigers defensive coordinator Brent Venables said. ''I don't know yet.''

The problems weren't there at the start of the season when Clemson's defense picked up the mantle left by last year's group, which ended the year ranked No. 1 overall in FBS in fewest yards allowed. Despite losing eight starters from a year ago, the Tigers showed out strong early on to pull out close victories over Louisville (20-17) and Notre Dame (24-22).

Clemson needed a last second stop on the Irish's two-point conversion try - defensive tackle Carlos Watkins stuffed Notre Dame quarterback Deshone Kizer - to leave soggy Death Valley with the signature victory.

The high water mark took place in Miami when Clemson shutout the Hurricanes 58-0 on Oct. 24.

Since then, the Tigers have shown breakdowns and inconsistencies defensively - and against several opponents they were supposed to dominate as easily as the Hurricanes.

North Carolina State put up 41 points against the Tigers, the most given up by Clemson in two seasons.

Syracuse kept things a game into the fourth quarter of Clemson's 37-27 victory before South Carolina made things too close for comfort to close the regular season.

''We wish we didn't have the breakdowns and played better,'' linebacker Ben Boulware said. ''It's something we will fix.''

A consistent defensive effort would help Clemson's offense, no matter how many points its capable of putting up.

Quarterback Deshaun Watson , third in the Heisman Trophy voting this year, led an attack that put up 60 touchdowns and 500 points this season. The offense seemed to find its full potency at the time Clemson's defense took its foot off the gas pedal.

The Tigers had nine straight games of 500 or more yards of offense to finish the year. Clemson's defense allowed 27 or more points in four of its last six games.

Clemson has been able to find enough balance on each side of the ball to build big leads down the stretch, but not to put teams away.

Against North Carolina, the defense forced two punts, intercepted Tar Heels quarterback Marquise Williams and stopped a fake punt try as Clemson went from trailing 16-14 to ahead 35-16.

Watson said the Tigers are ready to make whatever play necessary to win games - be it a long scoring pass or a forced fumble to stymie an opponents' momentum.

Venables called it a ''big plus'' that his guys get to work against Watson, 1,000-yard runner Wayne Gallman and a host of talented quick receivers. Venables said knowing how effective Clemson's offense is lets the defense be more aggressive, knowing that mistakes won't always cost the Tigers.

The defensive coordinator said, ''It's been fun watching those guys battle and go week in and week out against each other and have the kind of success that they've had on game day.''