Briton Murray has dropped a set in each of his first two rounds while Del Potro, who he beat in the Olympics final last year, has been playing through the pain after being doubtful because of back and shoulder injuries.
The Argentine 29th seed, who benefited from Nicolas Almagro's retirement in the second round, is satisfied with his level but said he could have pulled out of his previous match.
"He is, in my opinion, one of the best players in the world when he is fit and healthy," top seed Murray, who has had a disappointing season, said of the 2009 U.S. Open champion.
The Scot, however, is confident, even if Del Potro beat him in their last encounter at the Davis Cup semi-finals last year.
"I definitely feel like I'm capable of winning that match. I'm playing way better than I was two weeks ago," he said.
(Reporting by Julien Pretot; Editing by Ken Ferris)
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