Arizona star Derrick Williams used after-hours work at La Mirada High to become a prime-time player

The La Mirada High gymnasium would be closed for the night, but Derrick Williams didn’t let that stop him.

Wedging a bottle cap in the doorjamb, he found, worked wonders.

So he played on, for hours — sometimes with friends, sometimes alone, often with a coach who drilled him both for conditioning and on the finer points of the game.

Once in a while he’d get caught and scolded, but that didn’t deter him.

“Small price to pay,” says Charlie Torres, an assistant coach for the high school who usually came along. “Just look at him now.”

And see this: A 6-foot-8, 241-pound projected NBA lottery pick with the strength of a power forward and the ballhandling and shooting skills of a guard.

Williams was this season’s Pacific 10 Conference player of the year, and he’s averaging 19.1 points and 8.2 rebounds for a 29-7 Arizona team that has advanced to the NCAA West Regional this week at Honda Center in Anaheim. The Wildcats, the Pac-10 regular-season champions, play Duke on Thursday at 6:45 p.m. PDT.

Arizona’s two NCAA tournament victories have come by a total of three points, and Williams made key plays in the final seconds of both games. In a 77-75 win over Memphis, he made the go-ahead three-point basket, then blocked a shot that could have led to the tying basket with two seconds to play. And on Sunday, in a 70-69 win over Texas, he converted the game-winning three-point play with 9.6 seconds on the clock.

Williams, a sophomore, is smooth, polished and as comfortable firing three-pointers from the wing — he’s shooting 58.1% from beyond the arc — as he is banging bodies for rebounds under the basket.

Credit for that goes to those after-hours sessions at the high school gym with Torres, a 5-9 former community college point guard.

“Everything was guard work,” Torres recalls.

“Charlie spent hundreds of hours with [Williams] in the gym,” says Steve Schuster, now La Mirada’s head coach but then an assistant.

Torres also took Williams to pickup games across Los Angeles County — at Amelia Mayberry Park in Whittier, Lord’s Gym near Echo Park and in Compton, Downey and elsewhere.

“He knew I wasn’t going to score, was going to get banged up a lot, wasn’t going to get any fouls,” Williams said.

But the coach was committed to helping Williams grow as much in skill as he already had in stature — a tall order.

Williams was 5-9 around the end of eighth grade when his ankles and knees began hurting day and night.

By the end of ninth grade, he was 6-5 and had changed shoe sizes six times in about a year.

Still, it wasn’t until Williams’ sophomore year at La Mirada that college coaches began noticing him — and even that came by accident.

Tim Floyd, then USC’s coach, came to a game to watch one of his recruits, Compton High’s DeMar DeRozan, and came away impressed with Williams.

It was the start of a trend. In three games against high school teams with highly regarded prospects, Williams averaged 37.7 points.

Williams also played on a top-flight AAU team. Surrounded there by talent, Williams said he used the opportunity to “use other aspects of my game so I could score and show what I could do.”

He is now arguably the most efficient offensive player in college basketball, averaging nearly two points per field-goal attempt — largely because he’s shooting 60% overall and led the nation during the regular season with 289 free-throw chances, making 215.

Williams is also among the college game’s most exciting players, with a highlight reel full of spectacular dunks.

Even those are traced back to La Mirada, with Torres putting him through grueling leg resistance workouts.

As for how Williams ended up leaving Southern California and landing at Arizona, it’s an agonizing story for USC fans.

Williams grew up rooting for the Trojans, and he signed an early NCAA letter of intent to play for USC and Floyd.

But soon after, Floyd resigned amid an NCAA investigation that ultimately found former USC star O.J. Mayo had received improper benefits while in college.

Williams asked to be released from his commitment, and the school obliged.

Sean Miller, who had recently taken over Arizona’s program, scooped up Williams and two other USC recruits, Lamont Jones and Solomon Hill. Jones and Hill are Arizona’s second- and fourth-leading scorers; they and Williams combine to average 36.6 points, 48% of the Wildcats’ per-game output.

“The thing about recruiting is, you miss on some guys, and then sometimes you get lucky,” Miller says.

USC assistant Bob Cantu has another word he uses to describe what happened: “Painful.”

“We had him,” Cantu says of Williams.

Arizona may not have him after this season.

“He’s great, he’s a pro,” Kansas Coach Bill Self said after Williams scored 27 points in a loss to the Jayhawks at Lawrence in November.

Williams knows he’s considered a top NBA prospect, but being considered special is still something he’s getting used to.

“That’s pretty cool,” he says. “Going from people who really didn’t recognize me to going to different states and having them ask me for my autograph.”

Who knows, maybe La Mirada will even give him a key to the city . . . or at least the high school gym.

baxter.holmes@latimes.com