Both will join the publishing company in January and will report to chief agency officer Jane Wolfson.
Tsirkas is currently group client director at Initiative, responsible for clients including Amazon Fashion, Patek Philippe and Beko.
Morriss is currently agency group lead for WPP at Oath, before which she worked as head of agency sales at AOL.
Tsirkas has previously worked in TV, press and magazine buying and Morriss in cinema sales.
Wolfson said: "Sarah and Joni have a wealth of experience across multiple disciplines. Their strong commercial and strategic focus, coupled with excellent relationships, mean they will be a great addition to Hearst."