Serena Williams has admitted that she should have been eliminated from the Madrid Open after surviving three match points before beating Victoria Azarenka.

World number one Williams stared down the barrel of a first defeat in 2015 as Azarenka had one foot in the fourth round, leading 6-5 in the deciding set with a seemingly unassailable 40-0 advantage in the 12th game.

However, three successive double faults saw the Belarusian lose the game, before her American counterpart went on to snatch a 7-6 3-6 7-6 victory against the odds.

"I feel like the first two sets went by and then we ended up with this long third set," she told reporters.

"I could have won, she could have won and I ended up winning - I don't know how. Yeah I feel like it was intense. I don't feel that there were a lot of long points."

Williams will face Spain's Carla Suarez Navarro in the next round.