Juan Martin Del Potro advances into last 16 at US Open

Juan Martin Del Potro has made it through to the fourth round of the US Open, beating fellow Argentine Leonardo Mayer in straight sets at Flushing Meadows.

The 2009 winner won the opening two sets 6-3 7-5 in just over an hour before he called out the trainer for a knee problem.

With neither player dropping serve in the third, seventh seed Del Potro won a nervy tie-break 11-9 to seal his place in the last 16.

He will now face either Andy Roddick or Fabio Fognini in the next round.