The No. 1 seed in the tournament, Djokovic, of Serbia, needed just 72 minutes to defeat Nishikori behind three aces and five service breaks. And he had a message for No. 4 Alexander Zverev of Germany, his opponent in the semifinal.
"Matches are not getting easier, but my level of tennis is getting better and better," Djokovic told ITFtennis.com. "I know that I'm kind of a player that the further the tournament goes, the better I'm feeling on the court. That's the case here. (It was) my best performance of the tournament tonight against a very good opponent."
Zverev also cruised through the quarterfinals, topping Jeremy Chardy of France 6-4, 6-1 in 67 minutes.
The second semifinal will feature the sixth seed, Spain's Pablo Carreno Busta, and No. 12 seed Karen Khachanov, representing the Russia Olympic Committee.
Carreno Busta upset ROC's Daniil Medvedev, the No. 2 seed, 6-2, 7-6 (5). Khachanov outlasted No. 14 seed Ugo Humbert of France 7-6 (4), 4-6, 6-3.
Djokovic, who has won the first three Grand Slam tournaments of the year, needs to win the Olympic gold medal and the U.S. Open to join Steffi Graf of Germany (1988) as the only winners of tennis' Golden Slam.
Top seed Casper Ruud of Norway saved one match point in the second set and rallied to a 3-6, 7-6 (5), 6-1 quarterfinal victory over Sweden's Mikael Ymer on Thursday in Kitzbuhel, Austria.
Down 5-4 in the second set, Ruud saved match point, went on to win a back-and-forth tiebreak and then cruised through the final set. Ruud saved 11 of 16 break points in all while breaking Ymer seven times in 12 opportunities.
Ruud extended his winning streak to 10 matches. He has won two titles in as many weeks at the Nordea Open and the Swiss Open Gstaad and is now in search of a European clay-court treble.
The Norwegian will face Frenchman Arthur Rinderknech in the semifinals after Rinderknech upset No. 3 seed Filip Krajinovic of Serbia 6-4, 6-1.
German Daniel Altmaier and Spaniard Pedro Martinez will meet in the other semifinal. Altmaier outlasted Italy's Gianluca Mager 6-1, 6-7 (5), 6-3, while Martinez topped Slovakia's Jozef Kovalik 6-2, 6-2.
Truist Atlanta Open
Australian qualifier Christopher O'Connell advanced to the quarterfinals in Atlanta, upsetting second-seeded Jannik Sinner of Italy 7-6 (7), 6-4.
O'Connell, ranked 132nd in the world, saved three set points in the first-set tiebreaker before prevailing. He got the only service break of match to open the second set. It came on the lone break point of the match, as both players won more than 70 percent of their service points.
O'Connell's quarterfinal opponent will be sixth-seeded John Isner, who topped wild-card entrant Jack Sock 7-6 (6), 6-4 in an all-U.S. matchup.
Also moving into the quarterfinals were third-seeded Cameron Norrie of the United Kingdom and Emil Ruusuvuori of Finland.