Taylor Swift performs for Reese Witherspoon's 40th birthday

Reese Witherspoon celebrated her 40th birthday on Saturday (19Mar16) with a private performance by Taylor Swift and a duet with Keith Urban.

The Legally Blonde star marks the age milestone on Tuesday (22Mar16), but her family helped her get into the party spirit a little early by throwing a big bash in Hollywood this weekend, with a string of celebrity friends in attendance.

Guests at the event, held at the Warwick, included Kate Hudson, Matthew McConaughey, Robert Downey, Jr., Courteney Cox, Tobey Maguire, Jason Bateman, and Jennifer Aniston and her husband Justin Theroux.

Nicole Kidman was also spotted at the party, where her husband Keith was part of the night's entertainment, alongside surprise performer Taylor.

Birthday girl Reese, who arrived with husband Jim Toth, shared her excitement as she headed to the bash in a Snapchat video with her mum Betty, who was featured nodding as the actress asked, "Are you gonna get crazy on the dance floor, Mum?". She captioned the brief clip, "Crazy on the dance floor? #BirthdayWeekend #MomsInTown".

Meanwhile, pal Hudson also used social media to document the night's celebrations, including the moment when Taylor was introduced to the stage for her unexpected appearance, singing an acoustic version of Shake It Off.

"This beauty gave a little surprise for our bday (sic)," Kate captioned one Snapchat video.

"Happy birthday Reese, we love you!," she added alongside another.

Reese, who wore a sparkly silver dress, also got in on the fun by hopping onstage to join country star Keith, who played guitar for a rendition of Lynyrd Skynyrd's Sweet Home Alabama, which featured on the soundtrack to her 2002 romantic comedy of the same name.

"It was a great party," the source added. "When Reese left at 2am, she was in the best mood. You could tell that she had the best time celebrating her birthday."

Kate later returned to social media to share images of the festivities, making it clear she was exhausted after partying the night away, as she used the hashtags: "#TequillaFirst #PostingLater #SaturdayNight #ImSoExcitedToGetIntoBedIDontEvenKnowWhatToDoWithMyself #NotWashingMyFace #GoodNight (sic)".