WATCH the Japan Open LIVE in Australia on beIN SPORTS | Subscribe here and take advantage of our 7-day FREE TRIAL

Alcaraz, who overcame an injury scare in his straight-sets win over Argentine Sebastian Baez in his opener, showed scintillating form to dispatch Nakashima for his 65th win of the season, thumping 39 winners in the one-hour 20-minute contest.

The 22 year-old Spaniard, fresh from reclaiming top spot in the world rankings after clinching his sixth Grand Slam title at the U.S. Open earlier this month, next faces Norwegian fourth seed Casper Ruud.

Earlier, Ruud blazed past Australian qualifier Aleksandar Vukic with a clinical 6-3 6-2 win.

"These kind of matches help to keep my confidence really, really high. I played some great tennis," Alcaraz said.

"It's great to go through, to play another semi-final. It's special, because it's the first time I'm playing in Japan, here in Tokyo. To reach the semis in my first appearance is something great."

The biggest upset of the day came in Rune's 6-3 6-3 loss to American Brooksby, ranked 86th in the world.

"Once I get on a roll from a couple of matches, I just think I'm really able to get into my zone. I think that's one of my big strengths," Brooksby said.

"I thought I had a pretty good plan on him coming out here. I had never played him before, but I think that's good maybe, someone who hasn't faced my unique skills, I guess.

"I had a good rhythm from the first two matches and thought I was playing really good tennis, and I think I just stayed a little bit more consistent today."

Wimbledon semi-finalist Taylor Fritz overcame a second-set blip to beat Sebastian Korda 6-3 6-7(5) 6-3 in an all-American clash.

Second seed Fritz, who is set to climb to fourth in the ATP rankings, overtaking Novak Djokovic, will take on Brooksby in the semis.