Ice-cool Roger Federer showed nerves of steel to down Croatian giant Ivo Karlovic Friday as his great rival Rafael Nadal extended his hot streak at the Australian Open. As John Isner completed the worst ever US men\'s showing in the open era, Kim Clijsters moved one step closer to defending her title and Denmark\'s Caroline Wozniacki reached the fourth round as she clings doggedly to the top ranking. Injury-hit world number 11 Juan Martin del Potro equalled his best grand slam performance since winning the 2009 US Open, and Australian teen Bernard Tomic won a rollercoaster five-setter against Ukraine\'s Alexandr Dolgopolov. With the 6ft 10ins (208 cm) Karlovic hammering serves from on high and constantly coming forward, blocking the court with his outsized frame, Federer faced an awkward task. But at set point down in the first set tie-break and scrambling to a drop shot, he did not flinch and gamely aimed a vertical lob over the Croatian beanpole, who could only parry it into his own court. Federer, deftly combating the sport\'s fastest serve with his unerring backhand, took his first set point, then conjured the match\'s first break to take the second set before clinching it 7-6 (8/6), 7-5, 6-3. \"I knew it could come down to a few (points) here and there and obviously you need a bit of luck,\" he smiled. \"I definitely got a bit fortunate and I started to play better as the match went on.\" Earlier, Nadal bludgeoned his way into the fourth round with a straight-sets win over Slovakia\'s Lukas Lacko -- but he denied he was having it easy at the year\'s opening major. Nadal came into the tournament jaded and troubled by injuries, but he has given no such signs on the court so far. After a hard-fought opening against Lacko, the 2009 champion raced away with it 6-2, 6-4, 6-2. \"Nothing is easy. Everything is difficult. Every match you have your problems,\" he said. \"First set was 6-2, but I had to fight for it. It was a tough one. But I played solid all the match, so that\'s why I won.\" Clijsters, 28, showed her quality in a straightforward 6-3, 6-2 win over Daniela Hantuchova on Hisense Arena, as she aims to kick off her final season on tour with back-to-back Australian Open titles. And Wozniacki won 6-2, 6-2 against Monica Niculescu as she aims for her first major title and defends her top ranking from Victoria Azarenka, Maria Sharapova and Petra Kvitova, who could all be crowned number one after this event. The Dane, boosted by mental tips from her golf major-winning boyfriend, Rory McIlroy, took charge early against the Romanian and suffered her only lapse at 5-1 in the second set, when she was broken, before quickly recovering.  \"Whatever happens, happens,\" Wozniacki said of the number one ranking. \"I\'ve proven myself for the last two years. I\'ve finished number one twice in a row.\" Azarenka\'s temper flared when she wanted to contest a call on match point against Germany\'s Mona Barthel -- but she had run out of Hawk-Eye challenges, prompting a brief display of histrionics. After sealing it 6-2, 6-4 to stay unbeaten this year, the Belarusian third seed said she needed the extra emotion to get over the line. \"I had to get a little bit, not angry like in a bad way, just a little bit get my emotions going to finish the match,\" she said. Big-serving Isner was ousted in a five-set battle with Feliciano Lopez, meaning no American male has reached the round of 16 -- the worst showing in the open era, among editions of the Australian Open that US men have entered. \"It\'s very disappointing,\" said Isner. \"That\'s not a good effort from the Americans this tournament.\" Nicolas Almagro set up a fourth-round clash with Tomas Berdych while Nadal will play Lopez, and Federer will face 19-year-old Australian Tomic, who beat Dolgopolov for the first time backed by a roaring home crowd. Argentina\'s del Potro took out Taiwan\'s Lu Yen-hsun in straight sets and will face Germany\'s Philipp Kohlschreiber in the next round. Wozniacki will play former number one Jelena Jankovic, Azarenka faces Iveta Benesova and Clijsters will take on China\'s Li Na, who went through when Spain\'s Anabel Medina Garrigues rolled her ankle and retired in tears. Poland\'s eighth seed Agnieszka Radwanska will play Julia Goerges.