"I'm here with my best friend," Williams told a reporter at a New York screening for her latest film, Manchester By The Sea, this week.
"I'm so in love with her. She's proof that the love of your life does not have to be a man! That's the love of my life right there," she enthused of her longtime bestie.
Philipps is certainly feeling the love too - the Cougar Town and White Chicks actor shared side-by-side pictures of herself with Williams taken 15 years apart on Instagram this week.
The pair first met and struck up a friendship when they were both appearing on late nineties/early noughties TV hit Dawson's Creek.
"I love that we almost always stand on the same side of each other. Also, can I just say: we aren't at a costume party in 2001. That's legit how we were dressed for a regular night out," Philipps captioned the pic.
Philipps has been married to screenwriter Marc Silverstein for nine years, while Williams was in a three-year relationship with late Australian actor Heath Ledger which ended in 2007.
In a recent interview, she said that losing the father of her 11-year-old daughter Matilda in 2008 "won't ever be right."
"In all honesty, for pretty much everything else, I feel like I'm a believer in not fighting circumstances, accepting where you are and where you've been. In pretty much all senses but one," she told the magazine.
"I would be able to go totally down that line of thinking were it not for Matilda not having her dad. You know that's just something that doesn't ... I mean, it just won't ever be right," she said.
Despite the loss, Williams said she was proud to see she'd raised a happy and healthy daughter as a single mum.
"Not only are we OK, she's happy. Life has brought us to a place that's not just surviving, but thriving," she said.
