Chris Brown and Rihanna attend the 55th Annual GRAMMY Awards at STAPLES Center on February 10, 2013 in Los Angeles, California. Credit: Christopher Polk/Getty Images

Rihanna and Chris Brown walked away empty-handed from the Grammy Awards at L.A.'s Staples Center on Sunday (Feb. 10) -- but the on-again couple still felt like total winners in one another's company. During the evening of lively performances, Brown, 23, noticeably failed to join the audience in numerous standing ovations for the likes of Carrie Underwood, Kelly Clarkson and his nemesis Frank Ocean.

Notes one observer to Us Weekly: "The only time Chris Brown stood up the entire show was during Rihanna's two performances (her powerful ballad "Stay," and during a tribute to Bob Marley). Otherwise, he stayed seated." Indeed, although Brown and Rihanna, 24, didn't walk the red carpet together, they happily posed for several photos together inside the auditorium -- with Rihanna, clad in a stunning red Azzedine Alaia gown, cuddling with her man, wearing a white Lanvin suit.

The celebrating continued after the awards-fest -- four years after Brown's assault of Rihanna -- at the Zing Vodka Post Grammy Party held at Hollywood's Supperclub, where Brown served as host.

Having changed into a white outfit, Rihanna arrived past midnight and joined Brown at a private table near the DJ booth -- sitting in between Brown's legs, according to a witness. Sipping a cocktail and smoking a cigarette, Rihanna chatted with girlfriends and Brown danced in his seat to the music. Later, the pair sat side by side, whispering in each other's ears and "getting very affectionate," the observer says.

When the DJ played Rihanna's song "Birthday Cake" -- Brown recorded vocals on a remix version of the track -- the pair both sang and danced along. Escorted by their security team, the couple departed the bash around 2:30 a.m.