Utilize the input text as a context reference, choose the right relationship between Syriza and Alexis Tsipras from the options. Options: product/material produced, manufacturer, distributed by, industry, position held, original broadcaster, owned by, founded by, distribution format, headquarters location, stock exchange, currency, parent organization, chief executive officer, director/manager, owner of, operator, member of, employer, chairperson, platform, subsidiary, legal form, publisher, developer, brand, business division, location of formation, creator.

Having reluctantly agreed to stringent terms for a new bailout deal, Greek Prime Minister Alexis Tsipras must face down a rebellion in his anti-austerity Syriza party to push sweeping pro-market reforms and spending cuts through parliament.

Answer: chairperson