MELBOURNE (AFP) Defending champion Rafael Nadal was made to fight for a place in the Australian Open fourth round and join other title contenders Andy Murray, Juan Martin del Potro and Andy Roddick on Friday. Nadal claimed a hard-fought 6-4, 6-2, 2-6, 7-5 victory over German 27th seed Philipp Kohlschreiber to reach the last 16. The Spanish second seed was kept on court for three-and-a-half hours and will have a day to recover from the long physical match before taking on his fourth round opponent, unseeded Croatian Ivo Karlovic. Karlovic won the all-Croat encounter with 24th-seed Ivan Ljubicic, 6-3, 3-6, 6-3, 7-6 (9/7). Murrays serve was back in the groove as the Scot maintained his impeccable tournament form with a straight sets win over Frenchman Florent Serra. The 22-year-old equalled his best ever performance here with the 7-5, 6-1, 6-4 win, and has not dropped a set in three matches as he takes aim at a maiden Grand Slam title. The fifth seed, out to become the first British man to win a Grand Slam since 1936, will face giant John Isner in the next round, and then a potential quarter-final against world number two Nadal. Isner is playing really well and won the tournament last week, he said of the American who upset 12th seed Gael Monfils. The guy is 69" and has close to the best serve on the tour. Ive got to try to take care of that one first. US Open champion del Potro survived another searching examination to make his way into the last 16. The Argentine fourth seed emerged from a two hour 34 minute dogfight with Florian Mayer to down the 60th-ranked German, 6-3, 0-6, 6-4, 7-5. Del Potro will play 14th seeded Croat Marin Cilic in the fourth round. Cilic qualified with a 4-6, 6-4, 6-3, 6-3 win over Swiss 19th seed Stanislas Wawrinka. It has been rugged work for Del Potro at the years opening Grand Slam. He needed four sets to get past American Michael Russell in the first round and five sets to overcome another American James Blake in the second round. He has now spent almost 10 hours on court in his three matches, and should he get past the next round he could face American seventh seed Andy Roddick in the quarters. Roddick won a war of attrition with Spaniard Feliciano Lopez to claim a fourth round showdown with former finalist Fernando Gonzalez. In a grinding three-and-a-half hour contest, the American seventh seed proved the more consistent to register a 6-7 (4/7), 6-4, 6-4, 7-6 (7/3) win. Roddick, who is now 8-0 for the new season after his lead-up Brisbane tournament victory, will now play the Chilean 11th seed who was beaten here in the 2007 final by Roger Federer. Gonzalez came through a tough five-setter 6-7 (5/7), 6-3, 1-6, 6-3, 6-4 against Kazak Evgeny Korolev. Roddick, a four-time Australian semi-finalist, was relieved to get through to the round of 16 after dropping the opening set in a tiebreaker. I felt like I was real close to getting on top of the match and turning it into a little bit more comfortable. I just didnt quite get there, he said. But the only thing that matters, youre just trying to get through a day, and that happened, so no complaints.