Photo/IllutrationKei Nishikori waves after defeating Spain's Pablo Carreno Busta in their fourth round match at the Australian Open tennis championships in Melbourne, Australia, on Jan. 21. (AP Photo)

Kei Nishikori's fighting spirit could face the ultimate test at the Australian Open on Wednesday when he takes on world number one Novak Djokovic in the quarterfinals.

Nishikori overcame Spain's Pablo Carreno Busta in his third five-setter in four rounds on Monday and has spent nearly 14 hours on court at Melbourne, more than any other player.

Djokovic is aware of Nishikori's battling qualities having lost twice to the Japanese in 17 career meetings, including a defeat in the 2014 U.S. Open semifinals.

"He's a talented player. One of the quickest players on the tour, hard worker ... I expect a tough one," Djokovic, who beat Daniil Medvedev in four sets to reach the last eight, said.

"I have lots of respect for him. Every year is different, so every match that you play against each other is different ... I expect him obviously to come out, to try something new."

Djokovic, who took a tumble during his match with Medvedev, was also hampered by a back strain towards the end but shrugged off the problem.

"It was not particularly the fall. It was just a little bit of fatigue, a little bit of back. Nothing major. But there are a couple of things that have surfaced after a match like this," Djokovic added.

"I didn't feel so great in the last 20 minutes of the match or so ... we'll see how the body reacts, but I'm confident I can recover and I can be ready for next one."

Big-serving Milos Raonic, who cruised past Alexander Zverev in the previous round, takes on Frenchman Lucas Pouille in another quarter-final.

In the women's draw, Serena Williams will look to continue her progress but faces a potentially tricky opponent in Czech seventh seed Karolina Pliskova.

The American has dropped only one set so far in her quest to claim a record-equaling 24th Grand Slam.

U.S. Open champion Naomi Osaka meets Elina Svitolina.