All hail Stan Wawrinka, the new king of the U.S. Open.

Wawrinka pulled off a major upset in the finals Sunday, beating defending champion Novak Djokovic in four sets, 6-7, 6-4, 7-5, 6-3.

Wawrinka was relentless throughout the match, beginning in the first set. While he lost the opening game, he came back from a 5-2 deficit to force a tiebreaker that Djokovic ultimately won.

Wawrinka's steady play and ability to win the long rallies that are usually Djokovic's domain were the difference in the match.

CHAMPION! Stan Wawrinka upsets Novak Djokovic in a 4-set stunner for his 1st career US Open title. pic.twitter.com/rZv90ns4aP

— SportsCenter (@SportsCenter) September 12, 2016

Djokovic was slowed by a toe injury and leg cramps, but he fought hard throughout. He gained a love-30 lead on Wawrinka's serve in the final game of the match, but he could not capitalize and extend the fourth set.

Wawrinka has won all three Grand Slam finals in which he's appeared during his career, including the Australian Open in 2014, the French Open in 2015 and this year's U.S. Open.

Djokovic, gracious in defeat, called Wawrinka the "more courageous player" in his on-court interview with ESPN's Tom Rinaldi. He also described Wawrinka as "tougher mentally."

@stanwawrinka #USOpen pic.twitter.com/6KCAm2ApPp

— ESPNTennis (@ESPNTennis) September 12, 2016

Wawrinka said the two weeks of U.S. Open competition had drained him, and the Switzerland native thanked his team, acknowledging, "I'm not always easy, even if I think I am."

Amazingly, Wawrinka is 11-0 in his last 11 finals. He said he doesn't know why he's been so successful, but he pointed to his years of "hard practicing" that had steeled him for success.

Wawrinka started to put his signature on the match in the second set, as he jumped out to a 4-1 lead. While Djokovic battled back to tie it at 4-4, Wawrinka held serve to take a 5-4 lead and then broke Djokovic when the Serb superstar pushed a forehand wide.

The third set saw the two competitors tied at 5-5, and Wawrinka held serve to take a 6-5 lead. He then broke Djokovic again even though he fell behind love-30.

After Wawrinka took the third set, it seemed like only a matter of time before he would win his first U.S. Open title. Even if Djokovic had found a way to claim the fourth set, it would have been difficult to stop Wawrinka's momentum in the final set.

Djokovic, Roger Federer, Rafael Nadal and Andy Murray are known as the Big Four of men's tennis. Now that Wawrinka is 3-0 in Grand Slam finals, the Big Four may very well become the Big Five.