By Karolina Tagaris ATHENS (Reuters) - A grey-haired conservative with a steady "father figure" image has emerged as an unexpectedly strong challenger in Greece's upcoming national elections as voters grow weary of the charismatic young leftist leader Alexis Tsipras. Tsipras called early elections in August expecting an easy victory. The 61-year-old veteran politician and former speaker of parliament has kept a low profile since taking over the reins of the conservative New Democracy party in July as an interim leader, and was initially written off as no threat to Tsipras.