Piers Newbery
BBC Sport at Flushing Meadows

"The longest match of the tournament so far was Marin Cilic's four hour and 11 minutes spent beating Mikhail Kukushkin, and if Murray is to win this he will need considerably longer than that. With the on-court clock approaching three and a half hours, Murray is now closing in on a punishing 12 hours of court time. Potential quarter-final opponent Stan Wawrinka has spent nine hours and 23 minutes in action."

Something has tickled Kevin Anderson, who grins cheekily at the changeover before devouring an energy gel. He might well be happy after holding off Murray to edge back in front. The big South African's 21st ace of the match helps move him into a winning position, before a stray Murray backhand seals the deal.

How disappointing is that for Johanna Konta? She concedes the first set with a double fault after looking so confident for long periods against the fifth seed. Petra Kvitova saved three break points, but you just felt that she was starting to read the Konta serve. So it proved. First blood to the two-time Wimbledon champion.

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Less words, more action from Murray. The third seed keeps calm as he serves for the first time in this fourth set, dishing some punishment from the ball toss. Two booming aces clinch an easy hold to love.

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Stick your kettle on and grab yourself another cup of coffee - we're all in this together for the long haul. Anderson serves first in the fourth set of this match which is ticking close to the three-and-a-half hour mark. Murray looks shell-shocked as he is beaten by a delicate Anderson backhand winner which skims the net-cord and bounds to safety for 30-15.

Murray claws himself back to deuce as he gees himself up with some angry shouting. However, he cannot force a break point as Anderson hangs on.

Konta is jack-knifing into her serve. Crouch, toss, leap, belt. The first sign of pressure in this first set, not a great time when you're serving to stay in it. From 30-30, Kvitova can't keep the ball in the court, so we're heading into overtime.

Ros Clarke: Guess I'm staying up a little bit longer, then.

Konta is playing superbly but Kvitova saves two break points and eventually takes the game. Konta must serve to stay in the set.

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Do not go anywhere, Andy Murray is still in this. From Angry Andy at the end of the second, to Magic Murray here at the end of the third. He's still pointing to that group of three fans at one end of the court, as if he has made some new best friends tonight. If he wins, those guys will have tickets for the next round. We're going into a fourth.

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Anderson holds with a service winner at the second attempt. Murray matches him and is a pair of holds from winning the third set.

"Come on!!!" shouts Murray as he bounces on his heels after holding his serve. A bomb out wide cannot be returned successfully by Anderson who only finds the net.

Murray well in charge now as Anderson goes long. Two points ahead, the third set at his mercy.

Anderson's first serve is good, so is Murray's return. A long rally ends when Anderson wallops a stray forehand and Murray has a mini-break.

The British player trades games with her higher ranked opponent again. Full marks to her persistence. Konta not letting reputations get in the way.

Unreturnable from Anderson, Murray must unlock the monster South African serve to stay in this match.

The point of no return for Murray, the 2012 champion. Anderson wide with a backhand, on serve first up.

Murray challenges a crosscourt forehand which looks comfortably wide at 15-15. It is comfortably wide. Anderson leads 30-15 and then we have arguably the point of the match. Murray scurries across the baseline to keep returning, then going over Anderson's head. But he produces a ridiculous backhand winner to leave Murray shocked.

A tie-break is going to decide whether this match is going to be over or going into a fourth set....

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How are your nerves holding out? Andy Murray is hanging in there, keeping Anderson at bay to hold his serve and move within a game of the third set. Murray looks to the heavens as he tries to bypass the lanky Anderson with a lob....too long. That's 30-30 but Murray then he goes on the attack on the next point, springing forward to meet an Anderson return with a delicate drop-volley. And he forces the South African into a weak backhand to clinch the game.

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Konta has a very idiosyncratic bounce of the ball. Lots of elbow. It may be helping her serve, it may not, but she's rock solid at the moment. Another hold to love. In three service games, the Briton has lost only one point.