By Tony Jimenez TROON, Scotland (Reuters) - Jordan Spieth's decision to miss golf's return to the Olympics next month after a 112-year hiatus was the hardest of his life, the world number three said on Tuesday. The 22-year-old American adopted an appropriately respectful tone as he gave his reasons for missing the Rio de Janeiro Games. "This was probably the hardest decision I've ever had to make in my life," Spieth told a news conference ahead of the 145th British Open, which starts at Royal Troon on Thursday. More...