That thudding sound you hear? It's Kevin Federline's attorney smacking his head -- hard -- against his desk.

Seems K-Fed, who has sensibly kept his partying to a minimum as he tussles with former meal ticket wife Britney Spears for primary custody of Sean Preston, 23 months, and Jayden James, 11 months, decided to cut loose by whooping it up in Sin City.

On Friday night, just hours before his lawyer served a subpoena to Promises Treatment Center, where his disintegrating ex reluctantly received treatment back in February, Federline hit Las Vegas hot spot Tao with 25 of his closest hangers-on, reports People.

"He was drinking straight from the vodka and Jagermeister bottles," an eyewitness tattles to the mag. "I didn't see him with a drink in his hand that much, but he drank out of the bottles a few times."

But was it more than a few?

"He was slugging down Patron, Jagermeister and Grey Goose right from the bottle, gulping it down as if it were water," another spy maintains to Us Weekly. "It seemed like he just wanted to get wasted."

The failed rapper-turned-ostensibly-more-responsible-parent to Sean and Jayden (the chubby-cheeked tykes were reportedly with their mom over the weekend) also showed off some of his smooth moves with the ladies by chatting up a nearby table of lovelies.

"He was grinding up on a few of the girls," says People's mole. "He had a flashlight and was shining it on some of the girls around him."

(Come on, he was probably just using the light to draw attention to their bodaciously big and bouncy ... hair.)

As the night wore on, the lyrical genius who once rapped, "I got Gucci on, she got Prada, She calls me daddy, but she's not my daughter," repeatedly made a show of brandishing his gold necklace to the DJ, says the mag.

But his ersatz bad-assery didn't end there. K-Fed, whose baby mama ex Shar Jackson just declared to USA Today, "I can't speak about her parenting, but I know he's an amazing dad," held up a lighter when Jay-Z's "Big Pimpin'" played.

In the meantime, are things finally starting to line up for Federline, acting-wise? TMZ.com reports the blink-and-you'll-miss-him "CSI" guest-star has landed a one-shot spot on "One Tree Hill."

K-Fed was "extremely excited" about the North Carolina-based gig, but would only accept the role if the filming schedule worked with his custody arrangements.

Meanwhile, as her onetime spouse hit the Sin City scene and found gainful employment, Spears was making her own fun.

On Friday night, she slipped on Barbarella's wig, green contact lenses and a perilously plunging pink mini-dress to meet up with Criss Angel, her second rendezvous with the icky illusionist in two days.

Photo agency X17 says Angel cruised over to Beverly Hills, Calif., in his Rolls Royce to rendezvous with Spears, who earlier in the day was snapped sans ferociously fugly faux hair for the first time in months, smiling for shutterbugs from beneath a surprisingly healthy-looking blond shag cut.

They headed for nightclub Teddy's, but only stayed for about an hour before making a beeline back to Brit's pad -- with a quick stop at Taco Bell -- for the night.

But has the stubbly, diamond-draped "Mindfreak" star really conjured up a dalliance with the former pop starlet, who appears to be just one bad day away from another major meltdown?

Us reports he slept over on Friday, but Angel insists his late-night meet-ups with Spears are merely business.

He tells People he's helping her make her still-unconfirmed appearance at MTV's Video Music Awards next month "more innovative."

"It's amazing what people think," he marveled to the mag on Friday. "We're not together ... It's amazing how you can't do anything now without people making up stuff."

If that defense sounds familiar, it's because the PR-adoring prestidigitator, who turned up at a Playboy bash in Las Vegas on Saturday night, said something similar during his short-lived wooing of Cameron Diaz.

"It's crazy to me how the press takes one thing and blows it out of proportion," he huffed to Ryan Seacrest.

By the way, Britney's rumored VMA performance is news to the ceremony's musical director Timbaland, who tells "Access Hollywood," "I'm the maestro. If I don't know nothing about it ... could be, hope not. I don't know, I don't know nothing about it."

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