The blonde actress and the “Brothers & Sisters” star were later seen at San Lorenzo, a restaurant in the Knightsbridge area of the city, before heading to the exclusive Groucho Club on Thursday evening.
Balthazar, 33, reportedly flew into London on Wednesday to see the film actress.
Last week, Sienna won an out-of-court settlement totaling $79,000, from a photo agency whom she sued for harassment. Sienna filed suit earlier this year after Big Pictures, U.K., snapped her topless on a yacht with Balthazar over the summer.
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