No. 6 Novak Djokovic battled steamy conditions and a determined opponent but emerged with a 6-3, 6-4, 6-4 win over John Millman in the U.S. Open quarterfinals on Wednesday night.

Djokovic—who has not dropped a set in New York City since the second round—will now square off against No. 21 Kei Nishikori in the semifinals.

Millman, meanwhile, watched his Cinderella run come to a close two days after he stunned No. 2 Roger Federer in a four-set thriller.

However, Millman had no reason to hang his head despite bowing out in straight sets.

The unseeded Australian gave Djokovic all he could handle across a handful of extensive, entertaining rallies, and he made the 13-time Grand Slam champion work for every point in the early going.

With his legs working overtime in the heat and humidity, Djokovic showed real signs of fatigue that manifested themselves in the form of 53 unforced errors. (By comparison, Millman committed 46.)

Fortunately for Djokovic, a reprieve arrived at 2-2 in the second set when Millman asked for an unplanned break to change his clothes. Djokovic happily obliged and received an extended (and shirtless) breather that seemed to fuel him from that point forward.

The New York Times' Christopher Clarey relayed the U.S. Open's official statement on the strange stoppage:

Djokovic gained steam after his fuel reserves were replenished, and an early break in the third set effectively put Millman's hopes of another upset to bed.

Now safely through to the final four, Djokovic will set his sights on a semifinal showdown with Nishikori in which he projects as a heavy favorite. Dating back to 2010, Djokovic is 14-2 all-time against Nishikori, including a quarterfinal clash earlier this year at Wimbledon.

If Djokovic can survive one of the tour's toughest tacticians, he will square off against No. 1 Rafael Nadal or No. 3 Juan Martin del Potro in the final.

Stats courtesy of USOpen.org.