Reese Witherspoon is "amazed" by her daughter Ava Phillippe.

Ava Phillippe and Reese Witherspoon via Instagram (c)

Ava Phillippe and Reese Witherspoon via Instagram (c)

The 41-year-old actress took to Instagram on Saturday (09.09.17) to wish her daughter - whom she has with her ex-husband Ryan Phillippe - a happy 18th birthday, and said she "feels so lucky" to be a mother to the blonde beauty.

Posting a picture of the two of them together when Ava was a child, Reese - who also has son Deacon, 13, with Ryan as well as son Tennessee, four, with current husband Jim Toth - wrote: "Happy 18th Birthday to my sweet girl @avaphillippe ! Your kindness, grace, intelligence and humor never cease to amaze me. I love you and your HUGE [heart] and I feel so lucky that I get to be your mom. #proudmom #18 (sic)"

And Ava's father Ryan, 43, also took to Instagram to share a number of pictures of his daughter, each from a different stage in her life, and each captioned with part of a complete sentence.

Put together, the 'Shooter' star's captions read: "happy birthday, punk. i love you more than any strangers need read, but...

"you are the coolest


"most beautiful daughter a father could ever have

"and even though you're no longer a child, you'll always be my little girl. happy 18th, princess (sic)"

Meanwhile, Ava took to her own photo sharing account to post a picture of an envelope with her name on it, and revealed that her 'Big Little Lies' star mother had penned 18 letters - one for each of her birthdays - to mark her special occasion.

She captioned the snap: "18 letters that made me cry (hard) today, courtesy of my lovely mother. (sic)"