For Roddick, who surprisingly announced last week that he would retire following the year's final Grand Slam, his 13-year professional career came to an end with a forehand error.
"For the first time in my career I'm not sure what to say," Roddick told the Arthur Ashe Stadium crowd when handed the on-court microphone.
"Since I was a kid, I've been coming to this tournament and I felt lucky just to sit where all of you are and watch this game and see the champions that have come and gone.
"And I've loved every minute of it."
The 30-year-old Roddick began the match well, winning the rain-delayed first set in a tiebreak, but could not keep up with the towering Del Potro, who won the US Open in 2009, despite being roared on by a patriotic crowd.
With the win, Del Potro advanced to Thursday's quarter-finals against Serbia's defending champion Novak Djokovic, but the plaudits still went to an emotional Roddick.
Blessed with a powerful and a ferocious will to win, Roddick has been the torchbearer of American tennis since Pete Sampras and Andre Agassi retired.
He won the US Open, his lone Grand Slam title, in 2003 and briefly reached the top of the world rankings. He also made a second US Open final and three Wimbledon finals, but lost them all to Roger Federer.
With his wife and parents watching from the stands, Roddick struggled to keep his emotions in check as the realisation that his career was over began to sink in.
Del Potro embraced him at the net after the final point and Roddick wiped away the tears from his reddened eyes as the crowd stood to applaud the last American male to win a Grand Slam tournament.
"One thing I'm not scared about retirement is the people I go home to," said Roddick. "It's been a road, a lot of ups, a lot of downs, a lot of great moments."