Novak Djokovic tested en route to Australian Open quarter-finals as Lleyton Hewitt digs deep And when Djokovic faced another break point in the third game of the fourth set, the comeback looked set to continue. But the Serbian held his nerve to win the game and with that he regained the momentum. He claimed the Hewitt serve to go 4-2 up and it proved a lead he would not relinquish. "I have to give credit to my opponent who never gives up," said Djokovic. He's a great competitor and he kept on making me play an extra shot. I made a couple of unforced errors and he got back into the match. I have a lot of respect for him. For two sets and 3-0 I was playing really well and then I stopped moving but credit to him, he wasn't making any unforced errors. Next for Djokovic is a meeting with fifth seed David Ferrer in the last eight. The Spaniard was an impressive winner today, beating the dangerous Richard Gasquet 6-4, 6-4, 6-1. The other quarter-final in the top half will be between Andy Murray and Kei Nishikori after both men progressed today - albeit via different methods. Fourth seed Murray spent just 49 minutes on court before Mikhail Kukushkin succumbed to a hip flexor injury while Nishikori had to fight for three-and-a-half hours to get the better of Jo-Wilfried Tsonga 2-6, 6-2, 6-1, 3-6, 6-3. In the process, Nishikori became the first Japanese man to reach the last eight at this event in 80 years. Having reached the quarter-finals of a grand slam for the first time in his career, he said: "I am feeling unbelievable, to beat Tsonga makes me really happy. This is the first quarter-final for me, my best result previously was getting to the round of 16 at the US Open in 2008. I played well at the end of last year and now this. I feel like I'm stepping up. Nishikori is rapidly becoming a superstar in his homeland and results like this will only increase his popularity. "Hopefully it's big in Japan, I'm really excited," he added. Hopefully people, especially kids, will start playing tennis. But first of all I have to play well and I have to give them good news. But if that helps Japan then I'm really happy. Sixth seed Tsonga, a runner-up here in 2008, admitted Nishikori's superior mobility had given him the edge. "It was tough, I had a good opponent today," said the Frenchman. I didn't play good tennis and I didn't control everything. It was not a nice moment for me. He's tough to play because he runs a lot and everything comes back. The drama of the Nishikori-Tsonga match was in stark contrast to the non-contest of Murray-Kukushkin. The Kazakh beat Gael Monfils in five gruelling sets in the previous round and those exertions seemed to hinder him from the start as Murray cruised into a 6-1, 6-1, 1-0 lead before Kukushkin pulled out. "I played him a few weeks ago (in Brisbane) and it was a tough match," said Murray. I expected another one today but it's so hot. I get to conserve a bit of energy but he was obviously struggling. When I saw him struggling I was just trying to get a few serves in and get him running.