* Federer through to third Rome final * Nadal beats Berdych 6-2 6-4 * Serena sees off qualifier Halep (Adds Federer quotes) By Eric Salliot ROME, May 18 (Reuters) – Roger Federer will meet great rival Rafa Nadal in the Italian Open final after the Swiss held off a spirited challenge by unseeded Frenchman Benoit Paire on Saturday. The 7-6 6-4 scoreline suggested a routine victory for the world number three, but the 24-year-old Paire was a break up in the first set and held three mini breaks in the tiebreak before being let down by poor shot selection. …