The 11th-seeded Slovakian, who reached the last four at the Australian Open in January, struggled to find her range against the big-serving German and was swept aside in 71 minutes.

Groenefeld, world-ranked 141st after reaching a career-high 14th in 2006, kept her opponent on the back foot by dominating from the baseline and firing down seven aces.

The 23-year-old German will meet a wildcard opponent, either Australian Jessica Moore or American Melanie Oudin, in the second round.

Scare for Ivanovic

Meanwhile, top seed Ana Ivanovic survived a scare at the US Open on Tuesday before scraping past Vera Dushevina of Russia 6-1 4-6 6-4 in the first round.

The Serbian, chasing her first U.S. Open title, lost her way late in the second set in but bounced back to clinch victory in just over two hours.

French Open champion Ivanovic, who missed the Olympics with a right thumb injury, cruised through the first set and led 4-2 in the second before her game unravelled.

World number 57 Dushevina forced a decider and saved two match points but the top seed finally prevailed to set up a clash with Julie Coin of France or Australian Casey Dellacqua.

Blake’s night

Late on Monday night, ninth-seeded American James Blake was pushed to five sets by compatriot Donald Young before completing a 6-1 3-6 6-1 4-6 6-4 victory.

World No 1 Rafael Nadal also managed to shake off his Olympic hangover to register a hard-fought 7-6(4) 6-3 7-6(4) win over German qualifier Bjorn Phau.