Serena Williams fights back at US Open to keep calendar Grand Slam hopes alive

Serena Williams' calendar Grand Slam is still on after she battled back back from a set down to beat Bethanie Mattek-Sands in the US Open.

The American flirted with danger once again as she lost the first set to the world No 101 before recovering from her early wobbles to win 3-6 7-5 6-0 in the third round at Flushing Meadows. 

The world's top seed looked out of sorts at the start as fellow American Mattek-Sands twice broke the formidable Williams' serve to take the opening set.

But the defending champion showed her class with two breaks of serve of her own to level the match at one set all. 

With Mattek-Sands' resistance broken, WIlliams raced away with the decider, winning six games unanswered to reach the fourth round, where she will face another American Madison Keys. 

Williams remains on course to become the first player since Steffi Graf in 1988 to hold all four majors in a calendar year, but she insisted she is not feeling the pressure.

"I don't have to win this event. It's not the end of the world for me," Williams said.

"Of course it's there. I'm not a robot or anything. But at the end of the day I'm just here to do the best I can. If that means I win, then great. But if it doesn't, then you know what? I can't let that affect me because I still have other tournaments to play."

Her next opponent Keys had a more straightforward passage, beating 15th seed Agnieszka Radwanska in straight sets.

An all-Williams quarter-final remains on the cards after Venus secured a 6-3 6-4 win over Switzerland's Belinda Bencic.