Usher had a dance "battle" with Barack Obama at the president's 55th birthday party.

Usher

Usher

The 37-year-old singer had an "incredible" time at the bash, which took place at the White House earlier this month, and as well as singing an "impromptu" version of 'Happy Birthday', he spent a lot of time on the dance floor with the US leader and his wife Michelle Obama.

Usher told 'Extra': "Stevie Wonder, he performed a medley of songs and I came up impromptu and sang 'Happy Birthday'... It was a really amazing final birthday for President Obama at the White House and we had an incredible time.

"We danced all night... Me and Michelle always have a good time on the dance floor, but Barack, he actually battles you, right? It's like a toe-to-toe thing. There's no footage, though -- they took everybody's phone at the door!"

Usher can next be seen portraying boxing legend Sugar Ray Leonard in 'Hands of Stone' and he spent "about a year" getting in shape for the movie.

He added: "I grew my hair out... I did it all."

The 'Scream' hitmaker worked with a number of boxing pros to get in peak physical condition for the role, and refused to let them go easy on him.

He said: "I would tell 'em, 'Don't take it easy on me, let's go...' So I got rocked a few times."

While Usher already had some boxing experience before he began training, his co-star Edgar Ramirez,, who plays Sugar Ray's arch rival Roberto Duran, had never stepped into the ring before.

He admitted: "No, nothing -- that was my first time ever."

But despite Edgar's lack of experience, he left Usher impressed.

The singer said: "He kicked a*s. He did."

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