Saturday, 20 January 2018: World No.3 Grigor Dimitrov fears Nick Kyrgios's unpredictability ahead of their Australian Open fourth-round showdown at Melbourne Park.

Grigor Dimitrov fears the Nick Kyrgios X-factor as the pair shape up for a fourth-round Australian Open blockbuster on Sunday night.

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Seeded third behind only Rafael Nadal and defending champion Roger Federer, Dimitrov arrived at Melbourne Park as the challenger considered most likely to break the legendary duo's year-long domination of grand slam tennis.

But after barely surviving the first week, the Bulgarian knows he must raise his game to get past the in-form Kyrgios and keep his Open hopes alive.

Unbeaten in seven outings in 2017, Kyrgios's victory over Dimitrov in the semi-finals of the season-opening Brisbane International taught the world No.3 a valuable lesson.

Dimitrov fears Kyrgios can be unplayable at times and knows he must seize any small openings he's presented with at Rod Laver Arena.

"You never know what's going to happen (against Kyrgios)," Dimitrov said.

"First set, I thought I had everything under control. I was playing decent tennis and getting into some good rallies, but the second and third sets got away from me.

"You just have to always be on in the match. You never know when you're going to get an opportunity.

"Even after losing the second set, I thought I was going to get a little glimpse of opportunity during the (third) set.

"But then after that it was just impossible ... everything kind of slipped away from me."

While Kyrgios has been hugely impressive en route to the last 16, dropping just one set, to Jo-Wilfried Tsonga in a tense third-round affair, Dimitrov needed five sets to battle past little-known American qualifier Mackenzie McDonald and another four against Andrey Rublev.

But the fact the 2017 semi-finalist is still alive has Kyrgios on guard.

"I know it's going to be tough," said Australia's last man standing, who withdrew from the men's doubles on Saturday to save himself for an all-out assault on the singles.

"He loves the five-set format. He loves playing marathons. He's played well here in the past.

"I know it's going to be incredibly tough, but I'm not going to think about it right now.

"I feel like my level is there."

The victor will be rewarded with an unseeded quarter-final opponent - either Brit Kyle Edmund or Italian Andreas Seppi, who continues to prove an Open giant-killer.

Seppi last year eliminated Kyrgios from two sets down in the second round, after taking out Federer in 2015 and Lleyton Hewitt in 2014.

This time around, the 33-year-old withstood 52 aces from Croatian serving monster Ivo Karlovic to reach the last 16 for the fourth time.