Usually Serena Williams is the one doing the grunting. But during the tennis star's appearance on last night's "The Late Show," you'd be forgiven for getting that confused.

Ever the competitor, Williams just had to one-up her fashion from the 2011 ESPY Awards when she turned heads by wearing a formfitting, sexy hot-pink suit.

This time out Williams stunned everyone yet again, especially David Letterman, by arriving in a figure-hugging red dress. Letterman's reaction might best be summed up by this snippet of her chat with host Letterman.

Check out more pictures of Williams' outfit from "The Late Show" here.

"I think I saw your eyes almost pop out there," Williams said.

Responded Letterman, "Well, I think so ... That's not the only thing."

And while that's an image most American's could do without prior to bedtime, there's no question the outfit was a knockout. Victoria Beckham designed Williams' dress. Williams' six-inch heels were $4000 Swarovski crystal embellished Christian Louboutin Decorapumps. There's no word on who designed her diamond ring. The tennis champ finished off her look with her signature voluminous curly hair and flawless neutral makeup.

Of course, Wednesday night's Letterman appearance certainly wasn't the first time Williams has turned heads this summer. In July, she had the audience buzzing at the ESPY's because of that tight pink dress with a plunging neckline.

Williams, 30, considered one of the greatest players of all time, had an easy time showing up to Letterman because she's in New York this week preparing for the start of the U.S. Open on Monday. She discussed the gold medal she won at the Olympics, the damage tennis has done to her feet and why she plays better on grass than any other surface.

But even with the last Grand Slam of the season looming large Serena recently admitted on her website that she has a few off-court activities lined up while she's in New York City.

"I was just talking to my physio today about how the first thing I do when I get to New York is I'm going to karaoke,"  she said.

"I hadn't karaoked in, gosh, since like February or January. I'm a karaokier; normally I karaoke every day. ...Now I'm just overwhelmed. I have so many songs I want to sing"

Williams also confessed that her current karaoke obsession is Adele.

"I call it the Adele syndrome, not that I have an Adele voice," Williams joked. "My voice is so terrible!"

Expectedly, Letterman had to ask whether she'd consider playing tennis in her slinky red dress and heels.

"Wouldn't that be great?" Williams laughed. "Of course, I wouldn't get to any balls."

We're not going to touch that one with a 10-foot-pole. Okay, sorry.