Analyze the relationship between 'Syriza' and 'Alexis Tsipras' within the context of the input sentence.
Options: original broadcaster, employer, chairperson, brand

The vote, expected in the early hours of Friday, will test the strength of a rebellion by anti-austerity Syriza lawmakers, which could raise pressure on Prime Minister Alexis Tsipras to call snap elections as early as September.
chairperson