Losing the first set 4-6 was something of a recovery by Nalbandian after del Potro had stolen his first two service games for a 3-0 lead, but in the second set he was simply overpowered.

In the first match of the day, French world No. 16 Gilles Simon came back from two match points down to beat Robby Ginepri 6-7, 6-4, 7-6 in an enthralling 2 hour 36 minute duel against the American qualifier ranked 63 in the world.

On Wednesday, Roger Federer advanced to the third round with a 6-3, 7-6 win over Czech Radek Stepanek on his return from a three-week break.

Serbian third seed Novak Djokovic survived a searching test to beat Romanian Victor Hanescu.

Initially Federer had no such concerns, romping to a 3-0 lead as Stepanek's serve fell apart.

But the world No. 30 stepped up in the second set, winning all but one of the points on his first serve, and seized the advantage in the set decider with a sensational drop shot, before pumping his first wildly in celebration.

Two points later he had even greater impetus as he took a 5-3 lead but could not hold on as Federer applied the pressure to take the tie break 8-6 and win 6-3, 7-6 in one hour and 26 minutes.

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