Credit: Charley Gallay/Getty Images for SW

Kim Kardashian's new summer romance with Miles Austin keeps getting hotter!

On Monday, the reality star stepped out with the Dallas Cowboy at a private residence in Bel Air, Calif. Kardashian, 29, and Austin, 26, were guests at Serena Williams' Pre-ESPYs House Party.

Before heading out for the bash, a psyched Kardashian Tweeted: "Soooo excited for @serenawilliams party tonight!!! Woooowhoo"

The new couple spent their July 4th with another athlete-star couple: Kim's sister Khloe Kardashian and basketball star husband Lamar Odom. "Lamar and Miles are BFF now!" an insider tells Us.

The source adds that Kim is glad she took a chance on 6-foot-3 Austin, despite her worries about dating another football star on the heels of her breakup with Reggie Bush.

She showed her affection for her new man by throwing him a birthday bash at NYC steakhouse STK June 30. On the menu: vanilla cake with chocolate icing and strawberry rhubarb cobbler cocktails.

"[Kim] let loose!" a source tells Us of the birthday bash. Adds an STK observer: "Kim kept taking pictures of the two of them together."