The top-ranked Serb never gave Japan's Nishikori a chance in a 6-2 6-0 rout to reach the Tokyo Olympic tennis semi-finals on Thursday.

"He was defending amazing today - everything deep - and I was trying to stay with him but I couldn't," Nishikori said.

Djokovic has advanced into the medal rounds and - more importantly - moved one step closer to finishing off this segment of his golden slam bid.

In the women's tournament, 12th-ranked Belinda Bencic of Switzerland beat Elena Rybakina of Kazakhstan 7-6 (7-2) 4-6 6-3 to reach the gold-medal match where she will meet Marketa Vondrousova of the Czech Republic who beat fourth-seed Elina Svitolina of Ukraine 6-3 6-1.

Bencic also reached the doubles final with partner Viktorija Golubic.

Djokovic, who is in his third Olympic semifinal but has never made it to the gold-medal match, will next face Alexander Zverev of Germany, who beat Jeremy Chardy of France 6-4 6-1.

The other men's semi-final will feature Karen Khachanov of ROC against Pablo Carreno Busta of Spain.

Khachanov beat Ugo Humbert of France 7-6 (7-4) 4-6 6-3 and Carreno Busta defeated second-seeded Daniil Medvedev of ROC 6-2 7-6 (7-5).

Djokovic, who has already won the Australian Open, French Open and Wimbledon this year, needs the Tokyo Games title and the US Open trophy to become the first man to achieve the golden slam.

"Matches are not getting easier but my level of tennis is getting better and better and and I've done that so many times in my career," Djokovic said.

"I'm (the) kind of player that the further the tournament goes the better I'm feeling on the court and that's the case here."

Bencic became the fifth player to reach two finals at the same Olympics since tennis returned to the Games in 1988, joining Serena and Venus Williams, Andy Murray and Nicolas Massu.

The results mean Switzerland is guaranteed a tennis medal at a fourth consecutive Olympics after Federer and Stan Wawrinka won gold in doubles in 2008, Federer took silver in singles in 2012 and Martina Hingis and Timea Bacsinszky claimed silver in doubles in 2016.

Australian Associated Press