Flushing Meadows, New York, 2 September 2011 | AAP

Jelena Dokic was knocked out in the US Open second round by 2008 finalist Jelena Jankovic at Flushing Meadows on Thursday.

The Serbian won 6-3 6-4 in one hour and 21 minutes on Louis Armstrong Stadium despite needing an injury time out in the first set to treat a back injury.

Dokic broke Jankovic’s serve to open the match and held a 3-1 lead before the former world No.1 came back to capture a 4-3 edge.

Jankovic complained of back soreness at this point and had to leave the court for treatment.

On returning, she quickly wrapped up the first set.

World No.73 Dokic continued to try to stretch the 11th seed but the Australian paid the price for too many unforced errors, including 15 double faults.

Jankovic will play the winner of Petra Martic and Russian 17th seed Anastasia Pavlyuchenkova in the next round.

Bernard Tomic is in action later on Thursday against Marin Cilic while 29th seeded Australian Jarmila Gajdosova takes on Vania King.