BOISE, Idaho (Reuters) - World number one Novak Djokovic overcame an early ankle injury to lift Serbia to a Davis Cup quarter-finals victory over the United States with a comprehensive 7-5 6-7 6-1 6-0 victory over Sam Querrey on Sunday. I guess that's the biggest reason why I kept playing," Djokovic told reporters. Serbia won the tie 3-1, with Djokovic grabbing two of those victories.