06/27/2008 at 05:00 PM EDT
was back on stage in London Friday when she performed rousing renditions of two of her hits at a special concert as part of Nelson Mandela's 90th birthday tribute.
Wearing a white-and-black form-fitting dress, the Back to Black
singer, 24, sang "Rehab" and her massive hit "Valerie" for the crowd, who welcomed her with loud cheers.
Despite her recent hospitalization
, Winehouse looked healthy. She wore her trademark beehive loose at the back. Decorating her hair was a heart-shaped adornment inscribed with "Blake" for her incarcerated husband, Blake Fielder-Civil.
With Mandela watching in the wings, Winehouse danced while performing "Valerie" and revealed glimpses of her shorts-style underwear. At the end of the song, she made a simple bow of her head, moved back towards the band and thanked the Soweto Gospel Choir.
A surprise guest spotted in the crowd was Prince William's girlfriend Kate Middleton.
The first that the crowd saw of Winehouse was at about 8:30 p.m. when she joined the rest of the performers to listen to Mandela make a brief speech. The concert also raises awareness for his 46664 charity and campaign, named after Mandela's prison number. "Our work is for freedom for all," Mandela said.
Led by Will and Jada Pinkett Smith, Winehouse joined performers and guests such as Geri Halliwell, Leona Lewis and Annie Lennox on stage to sing "Happy Birthday" to Mandela, with Lennox blowing a kiss to the statesman as he left the stage.
Winehouse also took lead vocals in a version of "Free Nelson Mandela," alongside Jerry Dammers, the writer of the '80s anthem, and most of the concert's performers, including rockers Queen.