Madison Keys has said that she found it hard to play against a good friend in the fourth round at the Australian Open.

The American, who knocked out fourth seed Petra Kvitova at the weekend, beat compatriot Madison Brengle to progress to a Grand Slam quarter-final for the first time in her career.

The 19-year-old told reporters: "You know it's always hard just even being in this situation, then on top of it playing a friend."

Keys will face the winner of the match between Venus Williams and Agnieszka Radwanska in the last eight.