The Open and US PGA champion needed to win in Atlanta to secure the bonus, but having been overnight co-leader he saw his hopes fade with a double bogey and three other dropped shots on Sunday.McIlroy finished tied for second on eight under, with Horschel on 11 under.Fellow American Jim Furyk was also tied second, with Justin Rose tied fourth.World number one McIlroy made a double bogey on the sixth before three consecutive bogeys from the ninth, although he finished his round with three successive birdies and a par."It was a week too far," said the 25-year-old, who was competing in his eighth tournament in 10 weeks. "I was making mistakes that I don't normally make. But Billy deserved it. He played the best golf."The points-based FedEx Cup concludes with four play-off tournaments which take place after the regular 41-event PGA Tour season. Only the top 125 players, who accumulated the most points during the season, were eligible for the opening play-off event - The Barclays.That number was whittled down to the top 30 by the time of the finale at East Lake in Atlanta.Horschel fired his 12th consecutive round in the 60s to take the title, a first prize of £880,000 and the overall bonus of £6.2m.The 27-year-old graduate from the University of Florida was only 60th in the standings prior to the opening play-off tournaments, but finished runner-up in the second event, the Deutsche Bank Championship, before winning the next tournament, the BMW Championship at Denver."I'm on cloud nine, it's unbelievable," said Horschel."It's just unbelievable, everything that's happened the last two weeks, and especially having a kid on the way. I'm not sure life can be better than this."American Chris Kirk had led the overall competition going into the Atlanta tournament, but suffered a poor third round when he shot a 71 to drop from tied second to seventh.He eventually finished the Tour Championship tied for fourth on seven under alongside England's Rose and Australia's Jason Day.(BBC)Bakudaily.Az