Djokovic was leading 6-1, 4-1 when the eighth-seeded Nishikori let the umpire know he couldn't continue. Nishikori, the 2014 US Open runner-up, played three matches that went to five sets in his first four rounds. Two of those went to the new fifth-set super tiebreaker introduced at the Australian Open this year.
Nishikori needed a medical timeout for treatment on his upper leg after the first set. He wasn't comfortable throughout the match, and had his service broken four times.
"After third game or fourth game when I was serving, I felt pretty heavy to my right leg," he said. "After that I couldn't really bend my knees and couldn't jump up. Yeah, I decided to stop."
Nishikori said even if it wasn't somebody as formidable as Djokovic across the net, he didn't have enough in the tank to keep going.
"I was trying. Like I said, after couple games, I couldn't really move, couldn't hit my serve well," he said. "Yeah, I don't think even if it's Novak, I couldn't beat anybody with my one leg. It was just too tough."
Pouille entered the quarterfinal 0-3 against Raonic, including a first-round meeting here in 2016, but targeted the big-serving Canadian's second serve and won points on 35 of the 62 he faced. Pouille also protected his serve and was broken just once in the match.
"She's the right state of mind. She knows everything about tennis. It's not about being a woman or a man. It doesn't matter," Pouille said in an on-court interview when asked about the rarity of a top-ranked male player working with a female coach. "You just have to know what you're doing -- and she does."
"I didn't win a match in Melbourne before coming here" this year, Pouille said. "Now I'm in the semifinals. So just very, very happy."
The No. 28-seeded Pouille was up two sets and had break points in the seventh and ninth games of the third but Raonic held serve in clutch moments.
Raonic rallied late in the third, after getting angry when he challenged a line judge's call as Pouille was serving to stay in the set.
His service return hit the baseline but was called out. When he challenged it, and a replay confirmed his shot was good, chair umpire Nico Helwerth ordered a replay of the point.
Raonic thought he deserved to be given the point and, when he asked why it wasn't awarded to him, told the umpire "because you don't watch, because you're incapable."
Pouille won the replayed point and held serve for 6-6, but Raonic won the first six points of the tiebreaker and eventually forced a fourth set.
The set was on serve until Pouille put serious pressure on Raonic's serve and volleys, and he broke to clinch the match.
Raonic is now 1-3 in Australian Open quarterfinals, but he had a tough draw at Melbourne Park. He had to hold off Australian Nick Kyrgios in the first round, former champion Stan Wawrinka in the second and No. 4-seeded Alexander Zverev in the fourth.