Question: The Men's 2013 US Open Series was won by which Spanish player, currently World #1?
Related documents: [0]  [DOC] In tennis, the 2013 US Open Series (known as Emirates Airline US Open Series for sponsorships reasons) was the tenth edition of the US Open Series, which included ten hard court tournaments that started on July 20, 2013 in Atlanta and concluded in Winston-Salem for the men and in New Haven for the women on August 24, 2013. This edition consisted of four separate men's tournaments and four women's tournaments, with the Western & Southern Open hosting both a men's and women's event. The series was headlined by two ATP World Tour Masters 1000 and two WTA Premier 5 events. Rafael Nadal and Serena Williams were the US Open Series winners, making them eligible for a $1 million bonus if either also won the US Open, a feat which they both accomplished, hence receiving the largest paychecks to date for a single tennis tournament, totalling $3.6 million each. [END] 
[1]  [DOC] Rafael "Rafa" Nadal Parera (] , ] ; born 3 June 1986) is a Spanish professional tennis player, currently ranked World No. 1 in men's singles. Known as "The King of Clay", he is widely regarded as the greatest clay-court player in history. His evolution into an all-court threat has established him as one of the greatest tennis players ever. [END] 
Answer: 
Rafael "Rafa" Nadal Parera
Rafael "Rafa" Nadal Parera