After working so hard to keep his hopes alive here, Frances Tiafoe is immediately put under pressure in the fourth set as Alex Michelsen forces an early break point.

Tiafoe escapes, but a double fault prevents him getting over the line. He dusts himself down, wrong-footing Michelsen with a backhand, before his opponent sends a return long.

Relief for Tiafoe. Every single game feels crucial now.

Frances Tiafoe has won four and lost five of the fifth sets he has contested at the US Open.

The 22-year-old Alex Michelsen has lost all five of the fifth sets he has contested on the ATP Tour.

The stats tell us we probably have to fancy the experienced Tiafoe if the 11th seed can take it that far.

His attempts to make a positive start to another must-win set are stalled as Michelsen reaches deuce in the opening game.

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Frances Tiafoe brings up a second set point, and this time he takes it as Alex Michelsen mistimes a volley at the net.

Where has this come from? We're heading to a fourth set!

Frances Tiafoe plays the point of the match so far with a fine drop volley at the net which just spins out of reach from Alex Michelsen.

Michelsen is up against Tiafoe and the crowd at the moment, and after a brief fightback, he concedes set point by putting a forehand into the net.

This is pressure, but Michelsen rises to the occasion with a 133mph ace.

Francis Tiafoe is feeling it now as he serves only his second ace of the match, prompting another clenched fist in celebration from the 11th seed.

He then wraps up the game and Alex Michelsen must now serve to stay in the third set.

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Pam Shriver
Five-time Wimbledon doubles champion on BBC Radio 5 Live Sports Extra 2

What a signal Tiafoe has presented to Michelsen in this last couple of minutes!

There it is!

Alex Michelsen scoops a backhand long and Frances Tiafoe has the break back.

The 11th seed lets out a roar in relief and celebration.

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Alex Michelsen has dealt with pressure so well today but is he starting to feel it now? A double fault gives Frances Tiafoe a 0-30 lead.

Michelsen later plants a forehand long, and Tiafoe has two break points.

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That's got to be deflating for Frances Tiafoe to not be able to see that last game out.

He comes out swinging though, and outworks Alex Michelsen in a couple of short rallies, before sealing the game with a big first serve.

Michelsen will now serve for a place in his first Grand Slam semi-final.

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Alex Michelsen, with Frances Tiafoe at the net, somehow digs out a forehand which wins him the point.

Michelsen then secures the hold with another giant first serve.

Michelsen faced pressure there and responded brilliantly. He's one game away from the semi-finals.

Alex Michelsen saves the first two break points with a couple of big first serves.

He then rescues the third with a brave forehand which Frances Tiafoe can only put into the net.

Deuce.

At the start of the game Alex Michelsen steps inside the court to meet a volley but ends up putting it wide. Can Frances Tiafoe take advantage?

The crowd are hoping so, and they urge the 11th seed on as he wins the next two points to bring up three break points.

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Frances Tiafoe believes he can still get back into this as he follows up a well-constructed point with a loud "come on".

A perfect lob from the 11th seed then invigorates the crowd, before he seals the hold to 15.

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Alex Michelsen might be playing in his first Grand Slam quarter-final but he doesn't look like he's feeling much pressure.

The 22-year-old puts in another strong service game to hold to 15 and he's edging ever closer to the semi-finals.

Frances Tiafoe comes through a strong service game relatively untroubled.

The 11th seed can now have a breather at the change of ends, before trying to apply some pressure on Alex Michelsen's serve again.

The crowd are trying to lift Frances Tiafoe and they up the volume when the American 11th seed triumphs in a rally.

Tiafoe finds the pass next shot with Alex Michelsen at the net, but the world number 46 fights back brilliantly to secure the hold with some excellent defensive work at the back of the court.

Michelsen has proved tough to put away in the lengthy groundstroke exchanges today.

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Frances Tiafoe saves another break point before eventually coming through to hold, sealing the game with a backhand winner into the open court.

If Tiafoe lost that game you would have really struggled to see a way back for him.

Pam Shriver
Five-time Wimbledon doubles champion on BBC Radio 5 Live Sports Extra 2

We have to really give a lot of the credit to Michelsen, he's taken the serves and smothered Tiafoe.

Frances Tiafoe shrugs his shoulders as he puts a backhand into the net, before overcooking a forehand.

Tiafoe takes a deep breath at the back of the court before walking to the baseline to serve and despite getting the better of a rally, then offers break point to Michelsen as he goes into the net.

It would be a monumental mountain to climb if Tiafoe goes down two breaks here, but he wins the point to send the game to deuce.

Alex Michelsen serves his 11th ace of the match to get ahead, before Frances Tiafoe pulls a forehand wide.

Michelsen lets his American counterpart into the game with a double fault and Tiafoe takes advantage by forcing deuce with a strong forehand.

It doesn't come to anything, as Michelsen steps up with two strong first serves to secure the hold.