MELBOURNE, Australia -- Rafael Nadal's admiring followers had to be sweating a little bit Friday. Nadal needed four sets and 3½ hours to eliminate the all-guns-blazing Philipp Kohlschreiber.
Nadal meets another hard hitter, albeit a very different type of player, as the fourth round of the Australian Open begins.
Head-to-head: Nadal leads 2-0
A small gathering of fans hovered around an outer practice court at the Australian Open watching Ivo Karlovic deliver one booming, and accurate, serve after another in cool, breezy conditions Saturday. When the session was over, they raced to the timid 6-foot-10 Croat, hoping to get a picture.
He obliged, even waiting for one as he fiddled with his camera, causing a protracted delay.
Karlovic will have a much bigger audience Sunday, facing Rafael Nadal in an encounter sure to make both players anxious. No one enjoys playing Karlovic, as Frenchman Jo-Wilfried Tsonga pointed out less than sportingly last year, and the same can be said about the defending champion in Melbourne.
Karlovic reached the round of 16 at a Grand Slam for only the third time in 27 attempts, and for the first time outside the slick -- although slowing -- grass courts of Wimbledon. This, a month shy of his 31st birthday.
"It's a great feeling," Karlovic said. "I've been playing well the last three matches. Hopefully I'll continue to play as well."
Roger Federer, being as polite as possible, quipped at Wimbledon that dueling with Karlovic, who has perhaps the most unplayable serve the game has ever seen, doesn't constitute tennis. It comes down to a bit of luck, he added.
Nadal, 2-0 against Karlovic, seemed to agree. At the AEGON Championships on grass in 2008, they contested three tiebreakers. There were no breaks, the only two break chances went to Nadal, and Karlovic actually won more points.
"The match is decided in a few points," Nadal told reporters. "The thing is to be very concentrated on my serve all the time and try to convert the small opportunities that I can have on the return. That's it."
Many overlook that Karlovic is now an extremely solid volleyer and can smack the forehand, and still insist he's nothing more than a serve.
"I don't care," he said. "So long as I win, it's OK."
Karlovic, without a coach, spent time in Florida working with Tarik Benhabiles, Andy Roddick's former coach, during the offseason.
The next step for Karlovic is to beat a contender at a Grand Slam. His combined record against Federer, Nadal, Roddick, Novak Djokovic, Andy Murray, Juan Martin del Potro and Nikolay Davydenko is an unflattering 5-24.
His game plan against Nadal -- essentially his tactics against everyone -- is to "get in and be aggressive." Karlovic leads the tournament with 93 aces, 12 more than the 6-9 John Isner, although he's faced a hefty 31 break points.
Karlovic's best serving performance of the tournament came against countryman Ivan Ljubicic in the third round, registering a first-serve percentage of 71 and getting broken once.
Nadal had to crank it up Friday against Kohlschreiber, a wonderful shot-maker who wouldn't look out of place in the top 10. Nadal pulled out the fourth 7-5.
"For Rafa it was close, tough, but I think he was playing very good from the baseline, hitting hard," Spanish Davis Cup captain Albert Costa said. "He still needs to improve a little bit the percentage of the first serve."
Prediction: Nadal in four.
Head-to-head: Roddick leads 8-3
As Roddick's coach, Larry Stefanki -- who used to be in Gonzalez's corner -- put it, the Chilean despises big servers who can manufacture free points. Gonzalez struggled against slugging baseliner Evgeny Korolev on Friday, in a match that went five sets.
"It's tough to explain, because he was hitting every single ball as hard as he could," Gonzalez told reporters. "I felt really uncomfortable almost all the match."
Gonzalez can't possibly play as badly as he did against Roddick at the 2008 U.S. Open, a straight-sets drubbing. Gonzalez, who made his lone Grand Slam final here three years ago, will have plenty of Chilean fans behind him. The first set will be key for Gonzo.
Prediction: Roddick in three.
Juan Martin del Potro (4) versus Marin Cilic (14)
Head-to-head: Del Potro leads 2-0
Del Potro wasn't the only player to enjoy New York last summer. Cilic, who stands 6-6 like the Argentine and has a similar power game, reached his first Grand Slam quarterfinal after upsetting Murray.
When he encountered del Potro in the last eight, the Croat led by a set and 3-1 before dropping his level. He also took the first set off del Potro in the fourth round of last year's Australian Open.
Cilic began this year by defending his title at the Chennai Open. Although he was the heavy favorite, he stuttered versus 17-year-old Aussie wild card Bernard Tomic in the second round in Melbourne.
Prediction: Cilic in four.
Andy Murray (5) versus John Isner (33)
Head-to-head: First meeting
The tennis gods have smiled on Murray. His opponents, in order, have been South African qualifier Kevin Anderson, Marc Gicquel and another Frenchman, Florent Serra. He has breezed in straight sets, minus a small lapse against Serra.
The only real problem for Murray, one of the tour's best returners, was when he injured his back early against Serra. That's something to keep an eye on.
Isner is growing in confidence, coming off a first title in New Zealand last week and eliminating 12th seed Gael Monfils in the previous round. He packs Karlovic's service punch but is much better from the baseline.
"It's going to be a challenge to hold serve a lot," Isner told reporters. "That's going to be my No. 1 focus, holding serve and taking it from there. As long as I'm doing that, I know I'll be in the match."
Prediction: Murray in four.
Head-to-head: First meeting
Henin, tired following a grueling first week and nursing a slight leg injury, catches a real break facing her fellow Belgian. Wickmayer is even more damaged.
Wickmayer needed treatment to her back in a three-set win against plucky Italian Sara Errani on Friday and subsequently pulled out of a doubles match Saturday. Her health woes probably supersede an 11-match winning streak, including qualifying.
"Let's hope I can fix it up, yeah, by next match," said Wickmayer, the world No. 16.
Adding spice to the encounter is that Wickmayer -- who pummels the ball from the baseline and goes for returns -- briefly worked with Henin's coach, Carlos Rodriguez.
Prediction: Henin in two.
Head-to-head: Kuznetsova leads 5-1
Expect the unexpected from these two Russians.
More than a few must have thought Petrova, at some point, would revert to type and fade against Kim Clijsters in the third round. But Clijsters played so badly she didn't give the Russian a chance to collapse.
Petrova, armed with a big game and as high as No. 3 three years ago, says she's not content with simply upsetting one of the favorites.
"I'm not overexcited," she said. "I'm not super happy because I know the tournament is not over yet. I should stay focused and get ready for the next one."
Kuznetsova, the reigning French Open champion, makes seemingly easy matches look difficult, as evidenced again by a nervy performance in the third round against a German qualifier.
Petrova's head-to-head record against Kuznetsova is dismal, but then, she was 0-4 against Clijsters.
Prediction: Petrova in two.
Head-to-head: Safina leads 2-0
What a relief for Safina. No one considers her a favorite, and hardly anyone is asking her about her brother.
Similar to Murray, Safina has benefited from a comfortable draw. Her latest victim was Britain's Elena Baltacha -- improving but still only ranked 83rd -- in 57 minutes.
It was just what the Russian needed, since her offseason was disturbed by a back injury.
"The more matches I'm going to play, I'm going to try to do more and more things and feel more and more comfortable on the court," Safina told reporters.
Kirilenko, still seeking to fulfill her potential, ignited the tournament by upsetting Maria Sharapova. Just as impressive, the world No. 58 stayed mentally tough the next two rounds, not dropping a set.
Prediction: Kirilenko in three.
Head-to-head: Zheng leads 2-1
Could a Bondarenko be reaching a Grand Slam quarterfinal for the second straight Slam? Maybe. Bondarenko's sister, Kateryna, did it at the U.S. Open.
Bondarenko is on a roll, having won in Hobart last week to edge ahead of her younger sibling in the rankings. Along the way, she eliminated Zheng, a speedy baseliner, in a tight quarterfinal, 7-5, 7-5. On Friday, Bondarenko crushed eighth-seeded and serve-weary Jelena Jankovic 6-2, 6-3.
Prediction: Bondarenko in two.
Ravi Ubha is a frequent contributor to ESPN.com.