Easy for Novak at injury-hit Australian Open tennis Novak Djokovic had time for jokes and smiles as he knocked over a hobbling Nicolas Mahut at the injury-hit Australian Open Saturday -- but there were tears as two top women lost out. As a furious David Nalbandian vowed to fight his 'Watergate' fine, Djokovic did his best to cheer up struggling Mahut on his 30th birthday by happily playing to the crowd as he dispatched the Frenchman 6-0, 6-1, 6-1. Mahut, compromised by a left knee injury, maintained his good humour and he shrugged and smiled broadly to fans as he finally won his first game at the start of the second set. He was later given a birthday cake by reporters. Obviously, he had some strapping around his knee and I felt sorry for him," said defending champion and top seed Djokovic, who remains on course for a semi-final with Andy Murray. Mahut, who lost the longest match in tennis history at Wimbledon 2010, was not the only injury casualty on day six as Russia's Maria Kirilenko became the second women's retirement in quick succession. Just hours after Anabel Medina Garrigues agonisingly rolled her ankle against Li Na late on Friday, Kirilenko lasted until just game two of the second set against Petra Kvitova before retiring with a thigh problem. Wimbledon champion Kvitova took another step towards seizing the world number one ranking from Caroline Wozniacki, setting up a round four match with Serbia's Ana Ivanovic, who won in straight sets against Vania King. Maria Sharapova, who lifted the women's trophy four years ago, won 6-1, 6-2 against Angelique Kerber. But misfiring former world number two Vera Zvonareva departed in tears after she was shocked by 56th-ranked fellow Russian Ekaterina Makarova. China's Zheng Jie also upset France's number one, Marion Bartoli, 6-3, 6-3. Former finalist Jo-Wilfried Tsonga blazed into the fourth round for the loss of just six games against Portugal's Frederico Gil -- all without a coach. I've improved a lot," said the Frenchman. Fellow Frenchman Richard Gasquet also reached the fourth round and Spain's David Ferrer came through in three sets against Juan Ignacio Chela. And there were more fireworks from Nalbandian as he hit out against his hefty US$8,000 fine for splashing water at an official following his stormy exit in the second round. "During anti-doping control, I was washing my hands and the doctor incredibly accused to me of throwing water at him," he tweeted. In tempestuous scenes, Nalbandian had argued vociferously on court when he was refused a Hawk-Eye challenge at the climax of his five-setter with John Isner. It was another bad-tempered episode at the tournament after Marcos Baghdatis mangled four racquets in rapid succession in a stunning display of rage which quickly went viral on the Internet.