Defending champion Maria Sharapova is out of the US Open after a shock third round defeat to Agnieszka Radwanska at Flushing Meadows.

The Russian second seed was beaten 6-4 1-6 6-2 despite a winning sequence of seven games in the middle of the match which appeared to have put her on course for the last 16.

After struggling on serve in the opener, Sharapova dominated the second set and went 2-0 up in the decider, before 18-year-old Radwanska stormed back to win six consecutive games and earn the best victory of her young career.

The turning point for the 30th seed came in the third game of the third set, when she broke the Sharapova serve for the fourth time.

That gave her renewed confidence, while the doubts over her serve resurfaced for Sharapova, who was broken twice more before the conclusion of a fascinating contest.

After four breaks were traded in the opening set, a double fault from Sharapova gave Radwanska set point and she capitalised with a powerful backhand winner.

Sharapova made the first breakthrough in the third game of the second set and at last she appeared to have come to terms with the blustery conditions on the Arthur Ashe court.

She breezed through the remainder of the set and broke early in the third, but few could have anticipated what would happen once Radwanska had hit back.

The young Pole looked assured on serve and always threatened the Sharapova serve as the older player made 11 double faults.

The defending champion saved three break points at 4-2 down but was unable to save a fourth and Radwanska showed a steely nerve as she served out the match.

Sharapova said after the match: "Although I won many games in a row after losing the first and being down 1-0, you'd think I'd have the momentum in the third set with a break.

"But I just didn't really feel that way inside of me for some reason today."

"I could sit here all day long and come up with excuses and say how bad I played, but at the end of the day my opponent won the match and I lost it."