The ATP World Tour Finals—the season-ending championship of men's tennis—begins on Sunday, November 9 in London. Over the four days, we'll take a closer look at the seasons put forth so far by the eight singles competitors.

Whenever a major upset occurs in sports, we like to say that "the signs were there," or that "we should have seen it coming." I don’t think either can apply to Cilic's out-of-nowhere run at the U.S. Open. While he had plenty of wins this season heading into Flushing Meadows, he reached the semifinal round at just four minor tournaments—two of which were held in his native Croatia. We haven’t seen much of Cilic since; the Grand Slam champion passed on the Paris Masters after securing his passage to London. But it's impossible to forget how thoroughly he beat Tomas Berdych, Roger Federer, and Kei Nishikori en route to his first major title. And should Cilic collide with any of those men at the World Tour Finals, he'd be happy to remind them.

I’ll give you one guess. After outlasting Gilles Simon in a five-set fourth-rounder, Cilic won the final nine sets he played at the U.S. Open, and only one went longer than 6-4.

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At Wimbledon, Cilic had a chance to reach the semifinals for the first time, but lost the fourth and fifth sets to eventual champion Novak Djokovic, each by 6-2.

Cilic's championship at Flushing Meadows was no fluke; he's posted big results in the past, but never on a consistent basis. Following it up with another convincing showing would position the Croat as one of the preeminent players heading into 2015.

ATP World Tour Finals Previews

November 5: Marin Cilic

November 5: Roger Federer  
November 6: Novak Djokovic  
November 6: Milos Raonic  
November 7: Andy Murray  
November 7: Stan Wawrinka  
November 8: Tomas Berdych  
November 8: Kei Nishikori