The world No. 9 won 6-2 6-2 at Pat Rafter Arena on Saturday to set up a decider against Daniil Medvedev or Jo-Wilfried Tsonga as he bids for his first title in nearly three years.
But Nishikori, who had lost nine consecutive ATP finals over 52 tournaments in almost three years, broke the Russian player's serve on four consecutive points in the final game to clinch the match in just 2 hours, 6 minutes, at Pat Rafter Arena.
"A lot of my time is spent in the hotel lying in bed trying to conserve as much energy as I can, because whenever I step out on court there's so much I leave out there", he said.
"I was really nervous and he started fantastic", Medvedev said.
I played a great match against Dimitrov and today's match was another good tennis.
The other semi-final will be contested by Naomi Osaka and Lesia Tsurenko after the US Open champion managed to grind out a three-set win over Anastasija Sevastova while the Ukrainian dispatched Anett Kontaveit in straights.
The match then continued to go her way when Tsurenko rolled her ankle while down 1-0 in the third set.
"I'm just really happy with the way I played and I hope I will do better in the next round again".
The second set was a close affair and went to a tiebreak following 12 consecutive holds of serve.
The world number nine will enter this month's Australian Open with increased confidence having put an end to a frustrating few years.
"Physically and tennis-wise I think it was ideal".
"I don't really agree it was up and down because I didn't lose one serve in the match".
Medvedev, who beat Nishikori in the 2018 Tokyo final, was bidding for a 4th career title.
"I'm continuously learning and that's something that's never going to change for me", he said.
"I was a bit more aggressive than him and it made the difference", he said.
The 2017 Brisbane International victor claimed 14 of 15 points to force a deciding set, with Stubbs encouraging her to show more emotion.