Simona Halep wins battle with Lisicki to earn quarter-final against Victoria Azarenka

Romanian star Halep was forced to battle throughout against former Wimbledon finalist Sabine Lisicki, eventually coming through 6-7(6) 7-5 6-2.

Halep had chances to claim the opening set but was unable to take them, while Lisicki produced some of her best tennis of the year to push the world number two all the way.

After dropping the opening set Halep called out the trainer to have treatment on her left thigh and at the end of the second the players left the court when the heat rule came into effect before the exhausted Romanian prevailed in two hours 38 minutes.

In the second set Lisicki threatened to earn a shock win as the momentum swung her way.

Halep broke at 6-5 to claim the second set and then waltzed through a one-sided final set, with her superior fitness appearing to be the deciding factor as she made the last eight at Flushing Meadows for the first time in her career.

Her thigh heavily strapped, wounded Halep sensed her opportunity when Lisicki began to cramp in the third set, stretching between every point.

"It was a crazy match. I was fighting until the end, I did everything I could to win the match," said Halep.

It was the German's lack of consistency and unforced errors gradually crept up and overwhelmed her, while Halep was ruthless when it counted/

Halep converted 10 of 11 break points, and claimed the most crucial one at 4-2 before holding serve and breaking her 24th-seeded opponent again to clinch a last-eight spot at Flushing Meadows.

"It was a fighting match," added Halep. "But I'm really pleased to be in the quarter-finals."

In the earlier match 20th seed Azarenka beat Varvara Lepchenko 6-3 6-4 to secure her place in the US Open quarter-finals.

Lepchenko's powerful groundstrokes gave Azarenka something to think about throughout but the 20th seed's experience proved the difference.

After failing to serve it out, she broke Lepchenko in the final game to take the match.

The multiple Australian Open champion had both upper arms strapped during her match, but it did not appear to hamper her as at times she put on a brutal display of power hitting.

Flavia Pennetta also made the last eight after rattling through her match against Sam Stosur of Australia.

The Italian won 6-4 to make the last eight for the third year running, earning her seventh win in as many matches against Stosur.