Juan Martin del Potro spanks the forehand into Rafael Nadal's backhand.

The Spaniard can only slice into the top of the net for 30-30.

But that mini-chance is snuffed out before it has a chance to develop into anything more serious.

Rafael Nadal is darting around the court, looking up for the fight.

Juan Martin del Potro looks like he wants a cab home.

Jeff Tarango
Former professional tennis player on 5 live sports extra at Flushing Meadows

Del Potro has to hit aces - it doesn't matter if he double faults - and smash forehands so hard Nadal can't hit them back.

He's really late on the ball now, it's spraying off the racquet. It's like he can't focus on the ball properly.

Juan Martin del Potro stops the nine-game rot.

That was a game won through heart, mind and instinct rather than anything more pretty or creative.

David Law
Radio 5 live sports extra commentator at Flushing Meadows

Nadal is like a shark scenting blood in the water. Anything remotely short from Del Potro and he is on to it so quickly.

Juan Martin del Potro wafts a backhand into the net under no pressure to slip to 0-30.

I wonder if we might be close to a retirement from the big man.

He is running on vapours.

Rafael Nadal comes hustling into the net and whacks away a forehand for 15-40 and two break points.

Juan Martin del Potro survives via a big serve and a Rafael Nadal pass that is snagged, just, by the tape...

Deuce.

Juan Martin del Potro is looking like a man lost out there.

He picks at this strings, stares at his toes and can do nothing as Rafael Nadal takes the match by the scruff with a hold to love.

The Spaniard is skipping around the court, moving up and taking time away from the less mobile Del Potro.

And it is working a dream...

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Rafael Nadal's second serve return gives Juan Martin del Potro the hurry-up on his backhand side for 15-30.

And a backhand speared down the line like a harpoon! 15-40 and two break points for the Spaniard!

Into an epic rally, Back and forth. Both men off balance. And Nadal finds the route to victory, threading the ball against the grain and snapping the Del Potro serve once again.

Nadal's dad is off his feet and beaming...

Maybe the Juan Martin del Potro forehand is coming off the strings slower, perhaps Rafael Nadal has got up to the supersonic speed, but the Argentine's main weapon is not making as many dents as it was in the opening set.

Nadal holds to 15 and Del Potro is trudging.

He is a stroller around the court, but he looks even slower than usual out there.

Nadal has won the last seven games.

Jeff Tarango
Former professional tennis player on 5 live sports extra at Flushing Meadows

That set shows how much the first set took out of Del Potro. He was winded and looked tired after hitting all those forehands. I expected him to come out firing in the set but now I wonder if he has anything left in the tank.

Nadal has the momentum and he's unlikely to unclench those teeth.

Twenty-seven minutes was all it took for Rafael Nadal to dissolve the lead that Juan Martin del Potro worked so hard to establish in the first set.

Rafael Nadal is busier than a worker bee.

The Spaniard stalks in, moving up the court and dinking in a drop-shot for 15-30.

A second-serve return winner and Rafael Nadal has two set points.

Both clawed off the table - one via a streaking ace, the second by a duff return - and we are to deuce.

Rafael Nadal has won 20 of the 25 points in this set so far and is 5-0 up in 20 minutes.

It has been just one-way traffic.

Perhaps an fatigued Del Potro plans to win it in five by winning one, taking one off, repeat until victory.

Rafael Nadal has got the skids under Juan Martin del Potro.

The Spaniard slaps away a winner from mid-court for 15-40 and two break points.

Juan Martin falls on his sword with a terrible shanked forehand. He has come off the boil just as Nadal has turned up the gas.

Oh, and Hollywood institution Robert Redford caught one of the stray balls during that game.

Image source, Reuters

And a love service hold to send the players back to the stools.

No doubt who has made the faster start to the second.

Rafael Nadal is relishing a reset scoreboard. Juan Martin del Potro looks like his concentration has flickered since winning the opener.

A double fault from Juan Martin del Potro puts the big man in a spot of 15-30 bother.

What the serve takes away, it tends to give back threefold though.

A massive ace.

But Rafael Nadal is as tenacious as a terrier at the back of the court. He forces Del Potro into one more shot. Into the net. 30-40 and break point.

Del Potro tries to bully Nadal out of it, but he has overcooked.

Nadal clenches and shouts. First blood in the second set.

Piers Newbery
BBC Sport at Flushing Meadows

“We have a problem that when we hit our forehand to Del Potro’s backhand, our forehand is not enough…. good.”

One of the more intriguing innovations of US TV coverage is interviewing coaches during the matches, with ESPN's fearless Pam Shriver sent into the stands to mine for gold.

The ever affable Toni Nadal neatly sums up why world-class coaching can mean keeping it simple.

Image source, Getty Images

Rafael Nadal needs to rediscover some of that matador sass that he has at his best.

He looked a little dispirited by Del Potro's power towards the end of that first set.

That was more like it from the Spaniard as he moved Delpo around the court and pats away the backhand volley to hold to 15.