The World Group was the highest level of Davis Cup competition in 2016. The first-round losers went into the Davis Cup World Group Play-offs, and the winners progressed to the quarterfinals and World Group spot for 2017.
Great Britain vs. Japan
Murray's win over Nishikori was the joint longest match of his career at 4 hours and 54 minutes, along with the 2012 US Open final, until his davis cup semi-final tie with Del Potro.
References
2016 Davis Cup World Group Wikipedia(Text) CC BY-SA