Kiwi tennis No 1 Marina Erakovic has crashed out in the first round of the US Open in a defeat that will be more painful than she may care to admit.

Erakovic, the world No 54, was beaten 7-5, 6-4 by 19-year-old French wildcard Kristina Mladenovic to continue her dismal form at the US Open.

It is the third straight time Erakovic has been sent tumbling out in the first round in New York - though this loss, against a player nearly 100 places below the Kiwi on the world rankings, will be one that may haunt her.

It was a match that must go down as one of her most disappointing defeats of the year.

The 24-year-old has been struggling in her singles game of late but said before the tournament she felt she was "hitting the ball well" and confident she could turn things round.

She had won back-to-back doubles titles with Brit partner Heather Watson.

But it was not to be for Erakovic who was broken twice in each set and continued a flat patch of form that has seen her fail to progress beyond the second round of any tournament since Birmingham in June.

However she was not helped by a lower back injury that appeared to hinder her throughout and for which she required medical attention between sets. She confirmed afterwards she would have scans on the problem area tomorrow, and would likely have to withdraw from the doubles.

The opening set took 48 minutes as the Frenchwoman converted two of her four break-point chances, and won both the ace and winner counts 6-2.

The second set lasted an hour with the Kiwi falling behind 4-1 to an early break, battling back with a service break of her own but then unable to hold her serve in the 10th, and final, game of the set.

The Frenchwoman converted her second match point opportunity to make it out of the first round at a grand slam tournament for the first time in eight attempts.

For Erakovic it's a disappointing end to the year's major programme that saw her reach the third round at Wimbledon, the second round of the French Open and exit in the first round in Australia and now the US.

- © Fairfax NZ News