In a match characterized by lots of aggressive play from the back of the court, both players remained on serve in the second set until 5-5, when they curiously both lost successive service games, forcing a tiebreak in which del Potro ultimately prevailed 7-4.

"I think I played a very good match today," del Potro said afterwards. "Karen is a difficult opponent who has big potential to grow in the [ATP] ranking soon."

The Argentinian now faces a third round tie against Serbia's Filip Krajinovic, who bested Spanish wild card Feliciano Lopez 7-6, 6-3.

Women's no. 2 seed Caroline Wozniacki beat Petra Martic 7-5, 6-3 in a tense affair in the afternoon session.

The first set alone clocked in at more than 71 minutes, as both players dropped serve twice in what would turn out to be a grueling contest.

The set came to a climax at 6-5, with Martic having to serve to stay in the set. At 30-30 Martic approached the net only to see Wozniacki pass her with a brilliant cross-court backhand. The Croatian hit a backhand into the net on set point, dropping serve and in the process handing Wozniacki the first set.

Wozniacki stepped up a gear in the second set, dropping serve once, but also breaking her opponent's serve twice. The Dane started to find her range, and after upping her first serve percentage from 60 to 80 percent she was able to see off a tough opponent.

No. 9 seed Sloane Stephens suffered a surprise third round defeat to Slovakia's unseeded Dominika Cibulkova 4-6, 6-2, 6-3.

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