Karlovic served 59 aces in the match - but crucially just one in the deciding tiebreaker - as Nishikori prevailed 6-3 7-6 (8-6) 5-7 5-7 7-6 (10-7) on a hot and muggy day in Melbourne.

Nishikori broke Karlovic's serve just once in the marathon encounter lasting three hours and 48 minutes, but that proved too be enough.

"It was a really tough match and it could have gone both ways," said Nishikori, who has reached the last eight at Melbourne Park on three previous occasions.

"I was down 7-6 in the tiebreak but I focused well in the last points and I'm really happy to win."

The Japanese star did well to keep his cool when Karlovic made a successful challenge at a crucial moment in the super-tiebreak after his return was wrongly called out despite clipping the baseline.

The chair umpire then awarded the point to the Croatian after determining the linesperson's erroneous call did not affect Nishikori's subsequent shot which he dumped into the net, levelling the score at 6-6.

Nishikori will play German Philipp Kohlschreiber or Portugal's Joao Sousa in the third round on Saturday.

"I forgot that I was down 0-40 he almost had it for sure. Just one point, one serve and it could have gone his way," Nishikori said.

"I think this match will help for my confidence.

"He served really well today.

"I will try to carry on this confidence on for my next match.

"Almost my one year (of) aces, 60, never easy of course it's kind of frustrating if you cannot get the serve like three times in a row, but I think I focused well.

"Fourth set was really tough I lost four easy games.

"It was really fun on the court even though it was a really tough match I was really enjoying it I feel really good when I hear support."

Australian Associated Press