Sheer class! Jennifer Lopez ramps up the glamour in a classic black and nude gown as she joins beau Casper Smart at charity bash

PUBLISHED: 22:59 EST, 23 March 2013 | UPDATED: 11:12 EST, 24 March 2013

She regularly flashes the flesh in a striking barely-there ensemble.

But on Saturday night in Phoenix, Arizona, Jennifer Lopez opted for a more demure look in a stunning black and nude gown.

The 43-year-old star attended Muhammad Ali’s Celebrity Fight Night XIX in a gorgeous tulle Maria Lucia Hohan frock that largely covered her famous curves.

Stepping out with my baby! Jennifer Lopez took her boyfriend Casper Smart to Muhammed Ali's Celebrity Fight Night XIX in Phoenix, Arizona, on Saturday

Wearing her light brown hair long and loose around her shoulders, the former American Idol judge donned a halter-neck midi-length dress.

Layers of nude tulle sprouted from the black accent piece that nipped her waist and showed off her svelte figure.

To complete the ladylike look, Jennifer wore black and see-through striped heels, and held a metallic snakeskin clutch at the event, held at at the Marriott Desert Ridge Resort & Spa.

1950s fashion! The 43-year-old opted for classic glamour over a risque number for her date night

Quiet confidence: The usually over-the-top glamazon even took on a more demure persona, offering a small smile for cameras

Sure to never completely stray from her signature siren look, Jennifer’s bronzed skin sparkled in the cameras’ flashes and the dress’ black lace lining delicately framed her shoulders and décolletage.

Her toy boy beau Casper Smart, looked dapper in a navy blue suit and crisp white shirt.

The actress and singer was joined at the event by her sister Leslie Lopez who wore a striking floor length gown.

Fight night! The pair looked like the picture of Old Hollywood in a sweet dress and fitted suit

Sister act! Jennifer's night out was not complete without her sister, Leslie Lopez, who stunned in a red silk gown

Jennifer tweeted a picture of herself with Muhammed Ali Saturday night: '#THECHAMPISHERE @lyndalopez08 @celebfightnight #cfn'

Sweet tweets: Jennifer tweeted a picture of herself with Muhammad Ali Saturday night

Good spirits: Jennifer beamed as she sat down next to the boxer

Taking the stage: J-Lo gave a speech at the gala, appearing to introduce the boxer who was wheeled onto the stage

She also posed next to Muhammad Ali and Austin Powers star Verne Troyer, tweeting a photograph of herself with the legendary boxer.

'THECHAMPISHERE @lyndalopez08 @celebfightnight #cfn,' she wrote next to the snap.

The star took the stage at the gala which raises proceeds for the Muhammad Ali Parkinson's Center.

All for a good cause: Jennifer appeared to introduce the boxer after taking the stage and giving a short speech

Strike a pose: Jennifer has her photograph taken with actor Verne Troyer at the event

The event takes place annually and over 18 years, has raised more than $80 million for the charity, according to MyFoxPhoenix.

'I think it's a great cause, a lot of friends are here,' actor Steve Martin told the website.

The Dirty Rotten Scoundrels star took the stage with his banjo band at the gala.

Parents-to-be: Mario Lopez rubbed his wife Courtney Mazza's growing stomach - the couple have just revealed they are expecting their second child

An evening with the Champ: True Blood star Joe Manganiello arrived with stunning girlfriend Bridget Peters

He joined Mario Lopez, boxer Evander Holyfield, singer Andrea Bocelli, actress Rita Wilson, Bo Derek and True Blood actor Joe Manganiello on the carpet.

'You got to support the champ. Support the champ, support people who go through this horrific disease, what they do for one night, it's unbelievable,' actor Kevin Sorbo told Fox.

'The people are wonderful, it's for him and Lonnie and their amazing quest to put an end to this terrible disease,' said Billy Crystal, referring to the ex-boxer's wife.

Champagne time: Bo Derek looked stunning in a black lacy top, beads and blazer at the Marriott Desert Ridge Resort & Spa in Phoenix

Celebrity presence: Rita Wilson arrived in a glittery black dress and heels, while Steve Martin sported a dapper suit

Generous offer: An auction item at the event included a dinner with Billy Crystal, pictured watching proceedings and Robert De Niro

He's with the band: Martin played the banjo with the band Steep Canyon Rangers at the gala

There was also an auction at the event, including dinner with emcee Reba McEntire at her Beverly Hills home; dinner with Billy Crystal and Robert DeNiro; VIP tickets to a Paul McCartney concert; dinner with singer Andrea Bocelli in Tuscany, Italy -- and a round of golf with Phil Mickelson at The Bridges in Philadelphia.

Bo Derek added some more glamour to proceedings and was snapped sipping champagne in a black lacy top, beads and blazer.