Curlin made it close but finished second in his first test of the turf.
Red Rocks took the lead in the stretch and held off the reigning Horse of the Year by two lengths in winning the Man o' War Stakes at Belmont Park on Saturday.
Curlin dominated on dirt, winning nine times in 12 starts. This was a chance to see if the 4-year-old colt could stick with the top turf horses with an eye on running in the Arc de Triomphe, Europe's premier race, at Longchamp on Oct. 5.
Those plans are less certain now.
"We just didn't see that late kick from him today," said majority owner Jess Jackson. "I don't know if that was because he is still getting used to the turf or what. My feeling right now is that he needs another turf test. We know what he can do on dirt, and that is always an option."
Curlin, with regular rider Robby Albarado aboard, rallied in the stretch of the 1 3-8-mile race, but was unable to catch Red Rocks, a former Breeders' Cup Turf winner who came roaring past Sudan. The winning time was 2:12.60
Curlin, the 2007 Preakness Stakes winner, entered the race with five wins in a row, including the Breeders' Cup Classic and the Dubai World Cup.
Red Rocks, based in England, has now made three trips to the United States and run well every time. He competed in the last two editions of the Breeders' Cup Turf, winning in 2006 and finishing third last year.
"He hasn't been given the kudos he deserves since winning the Breeders' Cup," said trainer Brian Meehan. "Today, he confirmed he is one of the best middle distance turf horses in the world and has been for the last two, three years."
Javier Castellano was aboard for the first time as Red Rocks improved to 6-for-19 and earned $300,000 for J. Paul Reddam.
Red Rocks paid $14.40, $4 and $2.90. Curlin, the 2-5 favorite, returned $2.50 and $2.10. Better Talk Now paid $2.70 for show.
Sudan was fourth, followed by True Cause, Grand Couturier and Mission Approved.
Curlin picked up $100,000 for finishing second, improving his career earnings to $9,496,800 as he continues to close in on Cigar's all-time mark of $9,999,815.