Novak Djokovic has rediscovered his mojo to be ready to play the party pooper role as the Wimbledon men's championship heads towards an enthralling climax.

While tennis purists salivate at the prospect of another dream final showdown between Roger Federer and Rafael Nadal, the two living legends' greatest rival is threatening to tear up the script.

The only player with a superior head-to-head record over both grand slam giants, Djokovic has warned he's finally approaching his own brilliant best after an utterly unexpected two-year major title drought.

Even Djokovic admits his dramatic fall from grace after arriving at The All England Club in 2016 as the first man since Rod Laver in 1968 to hold all four slams simultaneously, only to rapidly relinquish them, had been mystifying.

"It was quite a strange feeling for me not to be able to deliver my game that I know I possess, that I know I've been delivering for so many years," Djokovic said ahead of Wednesday's quarter-finals.

"It was frustrating. But I had to trust the process. I'm still part of that process. But things are looking much better in the last month and a half."

Finally free off a painful and persistent elbow injury and blissfully content again in his private life, the Serbian plays another comeback star in Kei Nishikori for a drought-breaking semi-final berth.

The Japanese 24th seed is contesting his maiden Wimbledon quarter-final after finally overcoming his own wretched run with injury and backing up his third-round win over Nick Kyrgios with a fighting four-set victory over Latvian qualifier Ernests Gulbis.

"He's a big-match player," Djokovic said of the former US Open runner-up.

"He doesn't get affected too much by the big occasions. In contrary, he actually delivers his best.

"So that's what I expect him to do. Whenever he's fit, he can really beat anybody on any tournament.

"He has proven that in the past."

Chasing his third title on the hallowed lawns, the top-ranked Nadal faces Juan Martin del Potro in his first Wimbledon quarter-final in seven years.

The fifth-seeded Argentine completed a formidable quarter-finals line-up with an epic 7-6 (7-1) 7-6 (7-5) 5-7 7-6 (7-5) triumph over Gilles Simon on Tuesday.

Del Potro needed five match points to prevail in a match suspended on Monday night under fading light and stretched four hours and 24 minutes in duration.

Like Nadal, who could strike Djokovic in the semis, Federer hasn't dropped during his charge to a 16th Wimbledon quarter-final, almost surely a staggering record that will never be surpassed.

The Swiss maestro plays eighth Kevin Anderson, with the South Africa's 2017 Flushing Meadows finalist who is into his first quarter-final on his 10th visit to London's SW19.

Should Federer continue his relentless march towards a 21st grand slam crown, and ninth at Wimbledon, he is destined to meet a serving monster in the last four.

American ninth seed John Isner, another player making a career-best run on the grass, and Canadian 13th seed Milos Raonic - the runner-up two years ago - clash in a quarter-final featuring the tournament's top two ace leaders.

WIMBLEDON MEN'S QUARTER-FINALS ON WEDNESDAY (PREFIX DENOTES SEEDING):

1-Roger Federer (SUI) v 8-Kevin Anderson (RSA)

13-Milos Raonic (CAN) v 9-John Isner (USA)

12-Novak Djokovic (SRB) v 24-Kei Nishikori (JPN)

5-Juan Martin del Potro (ARG) or Gilles Simon (FRA) v 2-Rafael Nadal (ESP)