Former champion Juan Martin del Potro beat American Ryan Harrison in four sets in the second round of the U.S. Open.

The seventh-seeded Argentine won 6-2, 6-3, 2-6, 6-2 on Friday. The 20-year-old Harrison, ranked 61st, was seeking to reach the third round at a Grand Slam event for the first time.

Each player faced 10 break points, but del Potro converted seven to three for Harrison.

Harrison had 42 unforced errors.

Del Potro, who beat Roger Federer in the 2009 final, couldn't defend his title the next year after right wrist surgery. Del Potro lost in the third round last year.

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