A season that produced the biggest shakeup in men's tennis for more than a decade draws to a close next week with three ATP World Tour Finals debutants striving to cap breakthrough years with a large exclamation mark.

U.S. Open champion Marin Cilic, the man he beat in New York Kei Nishikori and Milos Raonic, the fresh-faced Canadian with dynamite in his serving arm, will all add plenty to the mix at the glitzy season-ender now in its sixth year of residence at London's O2 Arena.

However, three of the world's established "big four" will be only too happy to slap down the young upstarts.