Novak Djokovic and Roger Federer remain on course for a hotly anticipated Australian Open semifinal showdown. While Federer had little trouble breezing through his fourth round match in Melbourne on Sunday night, Djokovic was as taken the distance in a tussle far tighter than surely anyone would have anticipated.

France’s Gilles Simon has been a solid member of the world’s top 20 for much of the past seven years, but the fact that he had lost eight straight meetings with Djokovic just further illustrated the fact that he is not a player blessed with the weapons to worry the very best. What he did on Sunday, though, was to frustrate the world No. 1 into so nearly bringing about his own spectacular downfall.

Against a player showing off his unwavering consistency from the baseline and regularly striking the ball back down the middle of the court, Djokovic coughed up an extraordinary 100 errors. And so many of those came on break points, of which Djokovic won just six of 25. Every time it appeared Djokovic was ready to take charge, winning the first and third sets, Simon hung around long enough to strike back. In the end Djokovic took the final set 6-3, but only after being made to grind away for more than four-and-a-half hours on Rod Laver Arena.

“In terms of the performance itself, I haven't done well at all,” Djokovic conceded afterward. “You can expect unforced errors when you're playing Gilles Simon who is one of the best counter-punchers in the tour at the moment and he's been around for many years. He likes to play long matches. He likes to play long rallies. But I knew what was expecting me on the court. I honestly didn't expect to make this many unforced errors. In terms of a level that I've played, it's the match to forget for me.”

After making such a strong start to the year, where he picked up right where he left off in a dominant 2015, this was the first sign in some time of some potential vulnerability in the Djokovic game. That surely wouldn’t have gone unnoticed by Federer, as he watched on while preparing for his fourth round match later in the evening.

Federer was also taking on a player primarily known for his counter-punching skills, but had little trouble putting Belgian David Goffin away 6-2 6-1 6-4. There was little surprise that a large chunk of the questions put to the Swiss afterward concerned not his own straightforward win, but the struggles of Djokovic. Federer, though, was keen to downplay their significance.

“People just kind of talk maybe -- it's my assumption, I'm not surprised you're asking me,” he said. “I just feel people talking like he had a horrible day. Of course he can play better, but on the other side, you have somebody who has the fastest legs and he knows exactly what he's doing out there, and it worked almost to the very end. So it was very close for Novak, and he knows that.”

The interest is understandable, given that Federer and Djokovic have met in the final of the last two Grand Slams, with the Serbian halting Federer’s hopes of claiming an 18th Grand Slam title on both occasions. This time they are due to meet in the semifinal, and now just one round remains for both men to negotiate.

For Djokovic that task has been made harder by the physical exertions undergone on Sunday, and the fact he will be going up against another player capable of engaging him in long baseline rallies -- Kei Nishikori. Djokovic has won their last four meetings, but the Japanese seventh seed famously upset the 10-time Grand Slam winner in the 2014 U.S. Open semifinals.

Federer’s task is not a straightforward one either, going up against sixth seed Tomas Berdych. Like Djokovic, Federer has beaten his next opponent the last four times they have played, but 30-year-old Czech Berdych has come out on top previously at the quarterfinal stage at both the U.S. Open and Wimbledon.

The blockbuster rematch is now within touching distance, but Djokovic’s fourth-round travails just reinforce the fact that nothing can be taken for granted.