Britney Spears fans cringed as the pop star twirled around and gyrated to ex boyfriend Justin Timberlake's song.

The Grammy winning star tagged her former beau in her dance video. The song was the lead track on Jay-Z's 2013 album "Magna Carta... Holy Grail." It features vocals from the former *NSYNC singer.

Britney was dressed in a black turtleneck that had been cut from the bottom to show off her slim midriff.

She also had on a short pair of black and white plaid booty shorts that were partially rolled down.

The 39-year-old was dancing in front of the camera on her tiled floor, with a fully lit Christmas tree in the background.

Britney began by dancing in profile while gazing seductively at the camera while Justin's emotional vocals opened the song.

She could also be seen mouthing along with some of the lyrics.

As soon as Jay-Z entered, the song, the mom-of-two upped her energy levels to match.

"I KNOW ... turtle necks are SO ME!!!!," she gushed in her caption while tagging Justin's Instagram account.

Later, she edited her caption.

"Danced in my black [turtle emoji] neck last week to HOLY GRAIL !!!!" she wrote.

Several Instagram users commented on the video to express their confusion about the Christmas tree, her dance moves and why she tagged her ex.

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"Oh, dear. She doesn't really know what she's doing or she wouldn't be putting out videos with her eye makeup smeared the way it is," one fan noticed.

"Is that still her Christmas tree in the background? Its a shame I really feel for her, she looks so much happier and there in younger photos," another added.

"I don't think she's been the same since she broke up with Justin. She's still in love with him and remembers that time as her 'happy days,'" a third chimed in.

"So sad she used to be great dancer now she can't dance for toffee. This last year she's really not been right at all," a fourth wrote online.

Britney and Justin first met as Mouseketeers on The All-New Mickey Mouse Club, with her joining in 1992 and him following in 1993.

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The two fell out of touch for a few years after the show's 1996 cancellation.

However they reunited when Britney toured as support for *NSYNC in 1998. The singers struck up a romance later that year which would last until 2002.

After their break-up, Justin would release his solo break up-song "Cry Me A River" later in the year.

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