The Keeping Up with the Kardashians star has jetted off to Paris, where she met up with the tennis pro on Wednesday at Le Meurice, a luxurious hotel overlooking the Tuileries Garden, where suites can go for up to $20,000 a night.
The two are longtime pals – Williams, 32, attended Kardashian and Kris Humphries's August 2011 wedding.
After a quick outfit change, the pair headed over to Balmain, the venerated fashion house run by designer Olivier Rousteing – a friend of Kardashian's – where they spent about an hour. (Kardashian would later grab dinner with Rousteing at chic hot spot Ferdi.)
From Balmain, it was lunch at popular bistro L'Avenue, one of the reality star's favorite spots, followed by a quick stop at Häagen-Dazs, where a chauffeur grabbed two caramel-flavored ice cream cones for the ladies to enjoy on the way to their next destination: Lanvin.
At Lanvin, Kardashian, 33, was seen autographing her Vogue issue in the waiting area – the dress she wore on the magazine's cover was designed by the celebrated house's creative director, Alber Elbaz. Her presence at Lanvin didn't go unnoticed: A crowd formed outside, and as the reality star departed, a seemingly bemused Elbaz and two assistants were seen staring down from a window at the scene.
Following their visit to Lanvin, Kardashian and Williams hit the Marais and the showrooms of Azzedine Alaïa. (Back in January, Kardashian accompanied the legendary designer on a visit to the Galliera Museum, touring an exhibition there dedicated to his fashions.)
"So many exciting things are happening in my life," the bride-to-be told PEOPLE in March about her impending nuptials. "I feel so much good energy. It's just going to be the best!"