Del Potro had grabbed a 4-1 lead in the first set on the hard courts in Cincinnati when the Italian bowed out with a foot injury.
With the win, the Argentine set up a second-round match-up with Swiss superstar Roger Federer.
Del Potro defeated Federer in the 2009 U.S. Open final.
The Ohio event is the final ATP Masters tourney before the U.S. Open, which starts Aug. 29 in New York.
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