By Renee Maltezou ATHENS (Reuters) - Greek Prime Minister Alexis Tsipras's divided leftist Syriza party met on Thursday to thrash out its differences amid a mounting rebellion by far-left lawmakers who accuse the party of betraying its anti-austerity roots. Greece narrowly averted an exit from the euro zone for now when it struck an 11th-hour deal with lenders this month on a third financial rescue, but that has cost Tsipras the support of about a quarter of his lawmakers and put Syriza on the brink of a split. The deepening crisis within Syriza is the most serious political challenge to Tsipras, who otherwise enjoys unrivalled domination of the Greek political scene and remains popular despite his sudden U-turn to accept stringent bailout terms.