$ 0 0 World number one Rafael Nadal fought off a spirited challenge from Japan's Kei Nishikori to win through to the quarterfinals of the AustralianOpen on Monday. The Spaniard won 7-6 (7/3), 7-5, 7-6 (7/3) in three hours 17 minutes and will play Bulgaria's Grigor Dimitrov in the last eight. Nishikori, under new coach Michael Chang, pressed Nadal all the way and had his chances to take sets off the top seed only to fall down on forehand errors. Nadal was angered when he was called for a time violation by chair umpire Eva Asderaki at deuce while 4-4 at a critical time in the third set, but he regathered his composure to win the match in his second tiebreaker. The 13-time Grand Slam champion's renowned fighting qualities came to the fore when he won the big points for his sixth win over Nishikori. "He's a fantastic player and Kei is able to hit the ball very early and the ball was coming back very quick," said Nadal, who is now on a nine-match winning streak. "It was difficult for me to take position inside the court and I was close to losing every set so I congratulate Kei and I feel he will have a great season.