"So many peeps I love birthdays are today!" Cyrus, 20, tweeted Friday. "Gotta celebrate."
Music producer Williams turned the big 4-0 on Friday and celebrated the milestone with a party at Arkadia & LIV nightclub at the Fontainebleau hotel in Miami Beach.
Cyrus wore baggy black shorts a tight black crop-top that exposed her midriff for the occasion. "Girl in boys clothing"," she tweeted with photos of her ensemble. At the club, the "Can't Be Tamed" singer showed off her edgy style by posing for a picture with party guests Jennifer Hudson and Leah Labelle.
Posing with her hands in her pockets, it's unclear if Cyrus was wearing her engagement ring from Hemsworth, who didn't appear to be in attendance.
"So thankful for the birthday wishes," birthday boy Williams tweeted Saturday, "last night's party and all the artists who've ever allowed me to create music for them."
The next night, Cyrus got dressed up again to attend a bash hosted by pal Lil' Kim at Miami's Cameo nightclub. The LOL actress, who became engaged to Hemsworth in June 2012, partied from 11 p.m. until the club closed at 5 a.m., a source tells Us Weekly.
Cyrus, wearing a baggy black leather dress, was "having a great time dancing with a group of female friends," the source says.
Lil' Kim, 38, posted a picture of herself and Cyrus, whom she collaborated with on her track "Twisted," on the dance floor. She captioned the shot, "My twerk team came out to show me love at cameo tonight. Miley Cyrus and Teana Taylor I love you guys."