Top seed Novak Djokovic beat Spaniard Fernando Verdasco 7-5,2-6,6-2 to reach the quarter-finals of the men's singles of China Open today, keeping alive his hopes of clinging on to his world number one ranking.The defending champion, aiming for his fourth China Open title in five years after missing the 2011 tournament through injury, will lose the top spot next week if Spaniard Rafa Nadal reaches the final in Beijing.In the quarter-finals, the Serb will face American Sam Querrey, who defeated sixth-seed Swiss Stanislas Wawrinka 6-3 7-6.