It has finally cooled down at the US Open, but Novak Djokovic brought the heat to Kei Nishikori in Friday night's second men's semifinal, wearing down the 2014 Open runner-up with his stellar defense in a dominating performance. Djokovic a two-time US Open champ and 13-time Grand Slam winner, was just that much better than Nishikori in a 6-3, 6-4, 6-2 win to reach his record-tying eighth final in New York. 

It was a stark contrast from just a year ago, when Djokovic was absent from the final major of the season while trying rehab a nagging elbow injury that eventually required surgery. Now, coming off a triumph at Wimbledon in July, and with the more heralded -- and older -- Rafael Nadal and Roger Federer already out, another reign of the Djoker looks very real in men's tennis. 

Next up for Djokovic in Sunday's men's final: 2009 US Open champion Juan Martin del Potro. The two have squared off 18 times but never in a major final. Del Potro advanced earlier Friday when defending champion Rafael Nadal retired from their semifinal because of a painful right knee after dropping the opening two sets. Nadal is the first man to retire from a semifinal or final at the US Open since the professional era began in 1968.

Del Potro, the No. 3 seed from Argentina, is back in the final for the first time since winning the tournament in 2009. Nadal was clearly hobbled, and the overpowering game of del Potro ultimately proved to be too much. The final score was 7-6, 6-2.

The majority of the second set was largely played as a formality, with Nadal clearly not looking like himself after a hard-fought first set. He entered this match with over 16 hours of match time, including a marathon against Dominic Thiem in the quarterfinals. He was clearly frustrated that he had to retire, but the 32-year-old No. 1 seed looked uncomfortable all match.

Del Potro continued to dominate on his first serves, winning 70 percent of them. He also took advantage on a lack of power in Nadal's serve, winning 61 percent of Nadal's second serves.

Against Djokovic, del Potro is 4-14 in his career, including 0-3 in majors. A win would make this his best year in nearly a decade. He's made the French Open semifinals, the Wimbledon quarterfinals, and now a US Open final this year. In 2009, he made the Australian Open quarterfinal, the French Open semifinal and he won the US Open.

As for Djokovic, he looks to be completely back on top of his game after his slump while dealing with the frustrating elbow problems. 

"It means the world to me, obviously," Djokovic told ESPN's Tom Rinaldi on court after the win. " ... I'm trying to be the best I can be, every day, and obviously, I'm trying to reach certain heights that I guess I visualize. Every time I accomplish something big, I see a new mountain to conquer.". 

Sunday's men's final is set for 4 p.m. ET and will be aired on ESPN.