But Pitt and Miller were “heavily enjoying each other’s company,” and “they were talking to each other all night,” according to our witness.
It seems that their tête-à-tête has perked up the actor, who has been looking seriously thin during his divorce from Angelina Jolie.
Sienna and Bennett, who share the same surname and whose December birthdays are just two days apart, were alsp seen attending Wimbeldon twice together in July that year.
They remain 'best friends' despite splitting two years ago Sienna is dating Foxcatcher director Bennett after meeting on set of the 2014 movie, with sources confirming to Mail Online on Wednesday that they are still together.
In 2015, she spoke to Vogue about working with the director as she said: 'He opened the door to a world of filmmakers I didn’t have access to before.
Sienna had a well publicised affair with actor Balthazar while he was still married to fashion designer Rosetta Millington although he returned to his wife in the same year.
After the romance she said: 'I probably seem like not a particularly nice person, not a girl's girl' Sienna and actor Tom met in 2011 and became engaged the following year - shortly before she gave birth to daughter Marlowe Ottoline Layng Sturridge.
In 2015, Sienna spoke to Vogue about working with the director as she said: 'He opened the door to a world of filmmakers I didn’t have access to before. Everybody calls the last director you worked with, so it being Bennett Miller was a hugely beneficial thing.'Sienna separated from her former fiancé Tom Sturridge and has previously dated Jude Law. Meanwhile, in June the blonde beauty dismissed claims she is dating Brad Pitt , 53, after reports surfaced claiming they were 'exploring a casual summer romance' - in what would have been his first romance since splitting with Angelina Jolie , 42.
Despite the reports, sources told Mail Online the star is happily dating Bennett, with insiders stating: 'Sienna's finding it all very funny with friends have been texting her about Brienna laughing'.
Opting for comfort, the Alfie star completed her ensemble with a pair of white flatform trainers and accessorised with a black and white woven leather tote.
Smiling coyly, the British beauty seemed oblivious to the attention her dazzling ring commanded as she stepped out without her 50-year-old beau Bennett.
It seems to be the first public outing for the significant piece of jewellery, as Sienna wasn't spotted wearing the ring at a fashion party last week.
Mail Online has contacted a representative for Sienna for comment.
Sienna was at the festival as well and they hung out in a group that included their mutual friend Bradley Cooper, among others.