Nishikori, the fifth seed, cruised past the world No. 7 at Vienna's Stadthalle, winning 6-3, 6-1 to set up a semifinal with Kazakhstan's Mikhail Kukushkin on Saturday.
Cape Town - Kevin Anderson has become the first South African player in 23 years to reach the year-end ATP Finals.
Anderson, who has now won five ATP titles, said it had been his goal all year to qualify for the ATP Finals.
After Nishikori held a serve in a tightly contested first game, two Kukushkin returns into the net and a return ace saw Nishikori break in a love game.
Appearing in his fifth tour-level final of the season, Anderson captured his second trophy of the year after one hour and 55 minutes.
Anderson was runner-up to Djokovic at Wimbledon in July but the win in Vienna represented the biggest title of his career.
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The Japanese star triumphed 6-4 6-3 to improve his record against the Kazakh to 8-0. You have to win a lot of big matches against the best opponents in the world to book your spot there, so it's fantastic to be amongst the best eight players in the world.
NY and the 32-year-old's previous three career wins came in Johannesburg, Delray Beach and Winston-Salem were all at World Tour 250-level.
"The score might look easy, but the match was very tough", he said. I was up against an unbelievable opponent in Kei and I really had to play some awesome tennis. With Nishikori leading 5-4, Nishikori had two chances and converted the second set point.
The 28-year-old Japanese is making a late charge to the eight-player ATP Finals, in what would be his first appearance in two years when it opens in London on November 11.
A tense second set required a tie-break to split the two finalists, with Anderson finding his best level on his serve and forehand side.