Agnieszka Radwańska suffered a shock defeat to 20th seed Roberta Vinci in the fourth round of the US Open at Flushing Meadows last week.

The 23-year-old, who was seeded second for the tournament, lost 6-1, 6-4, as she was outplayed by her lower-ranked opponent.

After her defeat Ms Radwańska admitted that the Italian deserved the win. Asked how well she thought she had played, Ms Radwańska conceded, “Certainly not as well as I played in the initial phase of the tournament. Vinci was better. Roberta has a very hard style of play. … She had the answer to all my shots.”

For Ms Vinci it marked a career-best Grand Slam performance as she reached her first-ever major quarterfinal at the age of 29. In the first round she had also defeated Ms Radwańska’s younger sister, Urszula, 6-1, 6-1.

For the older of the two Radwańska sisters defeat to Ms Vinci was another setback, after she was recently knocked out in the first round of the Olympic tennis event in London in July. Ms Radwańska had been one of the favorites for both the US Open and London 2012 after she reached her first Grand Slam final at Wimbledon in June, before eventually losing to five-time champion Serena Williams.