American Jack Sock lost 7-5, 6-1 to unseeded Federico Delbonis in the first round of the Rio Open on Monday. The No. 6 seed, Sock lost in just one hour and 17 minutes.
No. 8 seed Thomaz Bellucci was also an upset victim, with the Brazilian falling in his home country to Alexandr Dolgopolove, 6-7(3), 7-5, 6-2.