Live coverage of the US Open will continue on BBC Radio 5 live sports extra until about 23:00 BST, but our live text commentary must come to an end, I'm afraid. We will return on Wednesday. Bye for now.
Latest results
There are other matches to talk about at Flushing Meadows. Ana Ivanovic is blowing kisses to the crowd after a 6-3 6-0 win over Alison Riske, and Sam Stosur has secured a 6-1 6-4 win over Lauren Davies.
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Despite that 54-minute defeat, it has been a productive year for Heather Watson who has introduced a touch of aggression to her game and returned to the top 50 after a difficult 2013. But Cirstea had too many weighty punches for the Briton today. On that showing, it'll be some time before the Briton reaches the second week of a Grand Slam. Well, 2015 at least.
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That was short and not so sweet. Watson's body language during the on-court interview gave the impression that the Briton wanted to disappear under a cloak of invisibility. Unfortunately, she had to face the music and admit that the better woman won.
It can't get much worse - Watson
Heather Watson on Sky Sports: "I just wasn't able to find my game because she didn't give me much time. It's another week, so I'll just look forward to next week now and the doubles. It can't get much worse so only moving up from here."
"It was a horror show from Watson in the second set. You cannot give Sorana Cirstea enough credit, she did not let up and Watson was not able to deal with the situation. Her game disintegrated in the second set. Her serve did not work for her at all. Eight double faults and to lose in 54 minutes is a big shock."
Watson, on the run, a flick of the wrist and the Briton bamboozles her opponent with a crackerjack of a forehand. Is there more of that on the menu? Nope. Watson collapses like a soufflé. Cirstea still ahead at 15-30 and, without much effort at all, she collects double match points thanks to a Watson double fault. A marvellous crosscourt winner from Cirstea and the Briton is out after just 54 minutes on the court.
"We were optimistic that Watson's steady progress up the rankings would continue, but Cirstea in the first set was magnificent with venomous hitting. A top five player would not play as well as that. Watson should not be too downhearted with the first set, but under pressure her game has deserted her in the second set."
A painful watch for Watson fans as the Briton is still unable to find the target. "Mother tennis can be so cruel," says Wayne Bryan in the BBC Radio 5 live sports extra commentary box.
With less than an hour played, Heather Watson - who had high hopes of reaching the second round - must serve to stay in the match.
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John McEnerney: This is going to be over soon unless HW gets her act together. SC is all over her like rash!
Laura Lambert: Don't doubt Watson's courage but has work to do. Needs to up her serve, and Cirstea's level will need to drop, to recover this.
Cirstea breaks
Watson 1-6 1-4 Cirstea*
A stooping Watson balloons a backhand long for 30-30 and the Briton needs to return to the aggressive game which earned her a place in the top 50 if she is to fight back from here.
Watson ends a lengthy rally with an overcooked forehand. To deuce we go. The forehand is Watson's enemy and so is her second serve - the 22-year-old winning only 14% of points on her second serve. Break point for Cirstea. The lights are flickering. A Watson backhand into the tape. Lights go out.
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*Watson 1-6 1-3 Cirstea
Watson performing like an out-of-control hosepipe, spraying groundstrokes here, there and everywhere. The match could be over before the Briton gains control of her racquet. She ends the game by going long. An easy hold for Cirstea.
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BBC Radio 5 live sports extra
Wayne Bryan: "I don't like to coach too much from the commentary booth, but Heather serves too many ball in the net. The court is a long skinny rectangle and, if anything, the more you serve long, the better. She is playing well enough on the ground, but the serve is letting her down, both in terms of percentages and doubles."
Cirstea breaks
Watson 1-6 1-2 Cirstea*
Watson opens with a double fault and that sets the tone for the next few minutes as the 22-year-old concedes three break points and just as we were thinking momentum was with the Brit, she coughs up another double fault.
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BBC Radio 5 live sports extra
Wayne Bryan: "Wow. Heather back to the haven of 1-1. That is an unbelievable, gutsy comeback because at 0-2 she was on her way out."
At 15-30, Watson has a sniff. The Briton crouches and blows at her fingertips at the baseline, as if to suggest she's prepared and ready for a showdown.
But Cirstea moves her opponent from side to side until Watson nets. Worry not, though, because Cirstea is fallible. A backhand into the tape - and Watson has a break back point! What has Watson got? Gravel for guts because she goes toe-to-toe with Cirstea and waits for her rival to crumble, the Romanian netting to gift Watson the break.
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BBC Radio 5 live sports extra
Wayne Bryan: "Heather only has 50 per cent of her first serves going in. That won't do, especially against a big server like Sorana."
Cirstea breaks
Watson 1-6 0-1 Cirstea*
It's time for Heather Watson fans to hide beneath the covers. The Briton's serve is faltering - only 50% have found the target - and she presents her opponent with two break points, throwing in the towel with a clumsy netted backhand. A horror of an opening game.
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Piers Newbery
BBC Sport at Flushing Meadows
Watson looks a bit stunned by this, it's been a terrific start from the previously out of form Cirstea, hammering down aces and clipping the lines. A bit of cloud cover gives brief respite from the sun, Watson could do with a lull at the other side of the net.
Wayne Bryan, father and coach of the Bryan brothers: "At 1-2 both girls looked strong, but one double fault from Heather cost her a service game and Sorana was off to the races. She will play better in the second set and it should be a lot tighter."
Can Heather Watson come back from a set behind? The Briton was hoping to reach the second week at Flushing Meadows for the first time in her career?
"It's so much of a target," said Watson, who could meet Eugenie Bouchard in the next round if she progresses today. "It was my goal this year and I haven't accomplished that yet, but I still have one more opportunity."
You can tweet your tennis chit-chat using the hashtag #bbctennis. There won't be any fanfare on their arrival (cutbacks), but they will be warmly welcomed on this grey August day.
Game and first set
*Watson 1-6 Cirstea
Cirstea has too much power for Watson, who plonks a return into the net to present her rival with three set points. Oof - but that's ugly from the Romanian as she overcooks an overhead. One set point disappears. But the Romanian classily ends the set with an ace.
Cirstea breaks
Watson 1-5 Cirstea*
If Watson is concerned at falling a break behind, she isn't showing it. At 15-0, the Briton double faults and then proceeds to give her opponent hope with a netted backhand for 30-30. Cirstea turns the screw, pouncing on a Watson serve to put the Briton on the backfoot and, ultimately, gain a break point.
Danger averted for Watson, though, thanks to a tasty serve out wide. The Romanian finding the lines, however, and another Watson double fault brings up a second break point. Eek. Watson holds firm, though - forehand crosscourt winner - and an ace takes her to advantage.
Third break point and this time Watson is blown away by her opponent's power, a backhand volley drifts wide and Cirstea closes in on the first set.
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*Watson 1-4 Cirstea
Cirstea overpowering Watson from the back of the court, although she did show weakness when double faulting at the opening point. The Romanian fires a forehand winner to merrily skip towards her chair at the changeover with a 40-15 hold.
Cirstea breaks
Watson 1-3 Cirstea*
It's a beautiful late morning in the Big Apple, and the compact Court 13 seems to be a delightful place to watch tennis, the sort of sporting arena where you are within touching distance of the players.
Both players turn the aggression-meter up a notch in this game. Cirstea with a pin-point accurate forehand for a break point and Watson meekly surrenders, coughing up a double fault at the worst moment.
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*Watson 1-2 Cirstea
A second serve ace down the 'T' from Cirstea for 40-15 suggests that the Romanian's serve packs a deceptive punch. She concludes a trouble-free hold with an ace and even though Cirstea weighs in at a feather-light 59kg, the 24-year-old's serve has a lot of muscle behind it.
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Watson 1-1 Cirstea*
Cirstea's radar needs readjusting, screwing a simple forehand wide to leak an easy point for 0-30. Big serving from Watson, the Briton cool and confident. An ace eases to within a point of a hold and another mammoth serve out to the wings wraps up an easy hold.
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*Watson 0-1 Cirstea
Watson immediately on the front foot, thumping a forehand return back towards Cirstea and the Romanian can only go long. First blow Watson, but the Romanian overhauls the deficit - a mixture of fine net play and a couple of Watson errors.
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The wait continues for the start of this match on Court 13, but the umpire has called "time" which means it should only be a matter of seconds until the match gets under way. Cirstea to serve first.
Who is Sorana Cirstea?
Watson's opponent, Romania's Sorana Cirstea, is ranked 80th in the world but reached a career-high 21 last summer. The 24-year-old's best performance at Flushing Meadows was in 2009, when she reached the third round.
In terms of head-to-heads, Watson has beaten the Romanian in two of their previous encounters.
More importantly, according to the WTA website, Cirstea describes herself as "ambitious, stubborn and happy." So there.
"I love New York, it's one of my favourite cities," Heather Watson told BBC Sport's Russell Fuller ahead of this clash.
"One reason is because I won the juniors here and every time I come back I use it as motivation. This is the first time I've been ranked above Cirstea, but I'm feeling very positive. This is a big opportunity for me now and I just want to make the most of it."
"People must think I'm crazy but if I don't believe in her then I have to say, 'OK, I think she can be 30 in the world.' And once she gets there it's like my job is done. No, let's go for more."
Watson on the rise
The cup of confidence is now overflowing for Watson after a difficult 2013 that saw a bout of glandular fever curtail her rise to a career high-ranking of 39 and, ultimately, led to her fall to 119 in the world by the end of the season.
But the Briton, playing more aggressively these days, has climbed 116 places in the rankings since February and is now back up to 45 in the world.
BBC Coverage
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Piers Newbery
BBC Sport at Flushing Meadows
It's a scorchingly hot morning in New York and Heather Watson has already been out on court getting a feel for conditions. She was hitting on Court 13 at 9.30am local time, with her opponent Sorana Cirstea on the other side of the net.
It's not unheard of that prospective opponents practice on the same court but it always feels a bit odd to observers. Cirstea has been struggling for form in 2014 and a former British number one watching from courtside had serious worries about the Romanian's forehand. "Heather needs to slice low to her forehand, she'll struggle to pick it up."
Post update
Hello! This is live text commentary of Heather Watson's first-round match against Romania's Sorana Cirstea. If you're looking for minute-by-minute updates on the Great British Bake Off, you've come to the wrong place. There will be no baking innuendos here.
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What a difference a year makes. Twelve months ago Heather Watson was dreading playing at the US Open. She was low on confidence, low on preparation and low in the rankings. But those tennis blues have melted away and the Briton is ready to make a brand new start of it, in old New York.
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Live Reporting
Aimee Lewis
All times stated are UK
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Latest PostPost update
Live coverage of the US Open will continue on BBC Radio 5 live sports extra until about 23:00 BST, but our live text commentary must come to an end, I'm afraid. We will return on Wednesday. Bye for now.
Latest results
There are other matches to talk about at Flushing Meadows. Ana Ivanovic is blowing kisses to the crowd after a 6-3 6-0 win over Alison Riske, and Sam Stosur has secured a 6-1 6-4 win over Lauren Davies.
Post update
Despite that 54-minute defeat, it has been a productive year for Heather Watson who has introduced a touch of aggression to her game and returned to the top 50 after a difficult 2013. But Cirstea had too many weighty punches for the Briton today. On that showing, it'll be some time before the Briton reaches the second week of a Grand Slam. Well, 2015 at least.
Post update
That was short and not so sweet. Watson's body language during the on-court interview gave the impression that the Briton wanted to disappear under a cloak of invisibility. Unfortunately, she had to face the music and admit that the better woman won.
It can't get much worse - Watson
Heather Watson on Sky Sports: "I just wasn't able to find my game because she didn't give me much time. It's another week, so I'll just look forward to next week now and the doubles. It can't get much worse so only moving up from here."
Post update
Russell Fuller
BBC tennis correspondent at Flushing Meadows
"It was a horror show from Watson in the second set. You cannot give Sorana Cirstea enough credit, she did not let up and Watson was not able to deal with the situation. Her game disintegrated in the second set. Her serve did not work for her at all. Eight double faults and to lose in 54 minutes is a big shock."
Listen to live coverage on BBC Radio 5 live sports extra
Game, set and match
Watson 1-6 1-6 Cirstea*
Watson, on the run, a flick of the wrist and the Briton bamboozles her opponent with a crackerjack of a forehand. Is there more of that on the menu? Nope. Watson collapses like a soufflé. Cirstea still ahead at 15-30 and, without much effort at all, she collects double match points thanks to a Watson double fault. A marvellous crosscourt winner from Cirstea and the Briton is out after just 54 minutes on the court.
Post update
Russell Fuller
BBC tennis correspondent at Flushing Meadows
"We were optimistic that Watson's steady progress up the rankings would continue, but Cirstea in the first set was magnificent with venomous hitting. A top five player would not play as well as that. Watson should not be too downhearted with the first set, but under pressure her game has deserted her in the second set."
Listen to live coverage on BBC Radio 5 live sports extra
Post update
*Watson 1-6 1-5 Cirstea
A painful watch for Watson fans as the Briton is still unable to find the target. "Mother tennis can be so cruel," says Wayne Bryan in the BBC Radio 5 live sports extra commentary box.
With less than an hour played, Heather Watson - who had high hopes of reaching the second round - must serve to stay in the match.
Get involved
#bbctennis
John McEnerney: This is going to be over soon unless HW gets her act together. SC is all over her like rash!
Laura Lambert: Don't doubt Watson's courage but has work to do. Needs to up her serve, and Cirstea's level will need to drop, to recover this.
Cirstea breaks
Watson 1-6 1-4 Cirstea*
A stooping Watson balloons a backhand long for 30-30 and the Briton needs to return to the aggressive game which earned her a place in the top 50 if she is to fight back from here.
Watson ends a lengthy rally with an overcooked forehand. To deuce we go. The forehand is Watson's enemy and so is her second serve - the 22-year-old winning only 14% of points on her second serve. Break point for Cirstea. The lights are flickering. A Watson backhand into the tape. Lights go out.
Post update
*Watson 1-6 1-3 Cirstea
Watson performing like an out-of-control hosepipe, spraying groundstrokes here, there and everywhere. The match could be over before the Briton gains control of her racquet. She ends the game by going long. An easy hold for Cirstea.
Post update
BBC Radio 5 live sports extra
Wayne Bryan: "I don't like to coach too much from the commentary booth, but Heather serves too many ball in the net. The court is a long skinny rectangle and, if anything, the more you serve long, the better. She is playing well enough on the ground, but the serve is letting her down, both in terms of percentages and doubles."
Cirstea breaks
Watson 1-6 1-2 Cirstea*
Watson opens with a double fault and that sets the tone for the next few minutes as the 22-year-old concedes three break points and just as we were thinking momentum was with the Brit, she coughs up another double fault.
Post update
BBC Radio 5 live sports extra
Wayne Bryan: "Wow. Heather back to the haven of 1-1. That is an unbelievable, gutsy comeback because at 0-2 she was on her way out."
Listen to live coverage on BBC Radio 5 live sports extra
Watson breaks back
*Watson 1-6 1-1 Cirstea
At 15-30, Watson has a sniff. The Briton crouches and blows at her fingertips at the baseline, as if to suggest she's prepared and ready for a showdown.
But Cirstea moves her opponent from side to side until Watson nets. Worry not, though, because Cirstea is fallible. A backhand into the tape - and Watson has a break back point! What has Watson got? Gravel for guts because she goes toe-to-toe with Cirstea and waits for her rival to crumble, the Romanian netting to gift Watson the break.
Post update
BBC Radio 5 live sports extra
Wayne Bryan: "Heather only has 50 per cent of her first serves going in. That won't do, especially against a big server like Sorana."
Cirstea breaks
Watson 1-6 0-1 Cirstea*
It's time for Heather Watson fans to hide beneath the covers. The Briton's serve is faltering - only 50% have found the target - and she presents her opponent with two break points, throwing in the towel with a clumsy netted backhand. A horror of an opening game.
Post update
Piers Newbery
BBC Sport at Flushing Meadows
Watson looks a bit stunned by this, it's been a terrific start from the previously out of form Cirstea, hammering down aces and clipping the lines. A bit of cloud cover gives brief respite from the sun, Watson could do with a lull at the other side of the net.
Post update
BBC Radio 5 live sports extra
Wayne Bryan, father and coach of the Bryan brothers: "At 1-2 both girls looked strong, but one double fault from Heather cost her a service game and Sorana was off to the races. She will play better in the second set and it should be a lot tighter."
Listen to live coverage on BBC Radio 5 live sports extra
Get involved #bbctennis
Can Heather Watson come back from a set behind? The Briton was hoping to reach the second week at Flushing Meadows for the first time in her career?
"It's so much of a target," said Watson, who could meet Eugenie Bouchard in the next round if she progresses today. "It was my goal this year and I haven't accomplished that yet, but I still have one more opportunity."
You can tweet your tennis chit-chat using the hashtag #bbctennis. There won't be any fanfare on their arrival (cutbacks), but they will be warmly welcomed on this grey August day.
Game and first set
*Watson 1-6 Cirstea
Cirstea has too much power for Watson, who plonks a return into the net to present her rival with three set points. Oof - but that's ugly from the Romanian as she overcooks an overhead. One set point disappears. But the Romanian classily ends the set with an ace.
Cirstea breaks
Watson 1-5 Cirstea*
If Watson is concerned at falling a break behind, she isn't showing it. At 15-0, the Briton double faults and then proceeds to give her opponent hope with a netted backhand for 30-30. Cirstea turns the screw, pouncing on a Watson serve to put the Briton on the backfoot and, ultimately, gain a break point.
Danger averted for Watson, though, thanks to a tasty serve out wide. The Romanian finding the lines, however, and another Watson double fault brings up a second break point. Eek. Watson holds firm, though - forehand crosscourt winner - and an ace takes her to advantage.
Third break point and this time Watson is blown away by her opponent's power, a backhand volley drifts wide and Cirstea closes in on the first set.
Post update
*Watson 1-4 Cirstea
Cirstea overpowering Watson from the back of the court, although she did show weakness when double faulting at the opening point. The Romanian fires a forehand winner to merrily skip towards her chair at the changeover with a 40-15 hold.
Cirstea breaks
Watson 1-3 Cirstea*
It's a beautiful late morning in the Big Apple, and the compact Court 13 seems to be a delightful place to watch tennis, the sort of sporting arena where you are within touching distance of the players.
Both players turn the aggression-meter up a notch in this game. Cirstea with a pin-point accurate forehand for a break point and Watson meekly surrenders, coughing up a double fault at the worst moment.
Post update
*Watson 1-2 Cirstea
A second serve ace down the 'T' from Cirstea for 40-15 suggests that the Romanian's serve packs a deceptive punch. She concludes a trouble-free hold with an ace and even though Cirstea weighs in at a feather-light 59kg, the 24-year-old's serve has a lot of muscle behind it.
Post update
Watson 1-1 Cirstea*
Cirstea's radar needs readjusting, screwing a simple forehand wide to leak an easy point for 0-30. Big serving from Watson, the Briton cool and confident. An ace eases to within a point of a hold and another mammoth serve out to the wings wraps up an easy hold.
Post update
*Watson 0-1 Cirstea
Watson immediately on the front foot, thumping a forehand return back towards Cirstea and the Romanian can only go long. First blow Watson, but the Romanian overhauls the deficit - a mixture of fine net play and a couple of Watson errors.
Post update
The wait continues for the start of this match on Court 13, but the umpire has called "time" which means it should only be a matter of seconds until the match gets under way. Cirstea to serve first.
Who is Sorana Cirstea?
Watson's opponent, Romania's Sorana Cirstea, is ranked 80th in the world but reached a career-high 21 last summer. The 24-year-old's best performance at Flushing Meadows was in 2009, when she reached the third round.
In terms of head-to-heads, Watson has beaten the Romanian in two of their previous encounters.
More importantly, according to the WTA website, Cirstea describes herself as "ambitious, stubborn and happy." So there.
Post update
BBC Radio 5 live sports extra
"I love New York, it's one of my favourite cities," Heather Watson told BBC Sport's Russell Fuller ahead of this clash.
"One reason is because I won the juniors here and every time I come back I use it as motivation. This is the first time I've been ranked above Cirstea, but I'm feeling very positive. This is a big opportunity for me now and I just want to make the most of it."
Listen to live coverage on BBC Radio 5 live sports extra
Can Watson be number one? Why not?
Heather Watson's coach has started spreading the word that his student could, one day, be top of the heap.
"Can she be number one? Why not?" said her Argentine coach Diego Veronelli.
"People must think I'm crazy but if I don't believe in her then I have to say, 'OK, I think she can be 30 in the world.' And once she gets there it's like my job is done. No, let's go for more."
Watson on the rise
The cup of confidence is now overflowing for Watson after a difficult 2013 that saw a bout of glandular fever curtail her rise to a career high-ranking of 39 and, ultimately, led to her fall to 119 in the world by the end of the season.
But the Briton, playing more aggressively these days, has climbed 116 places in the rankings since February and is now back up to 45 in the world.
BBC Coverage
BBC Tennis: Today at 4 o'clock. On your radio and on the @BBCSport website. Join us.
Post update
Piers Newbery
BBC Sport at Flushing Meadows
It's a scorchingly hot morning in New York and Heather Watson has already been out on court getting a feel for conditions. She was hitting on Court 13 at 9.30am local time, with her opponent Sorana Cirstea on the other side of the net.
It's not unheard of that prospective opponents practice on the same court but it always feels a bit odd to observers. Cirstea has been struggling for form in 2014 and a former British number one watching from courtside had serious worries about the Romanian's forehand. "Heather needs to slice low to her forehand, she'll struggle to pick it up."
Post update
Hello! This is live text commentary of Heather Watson's first-round match against Romania's Sorana Cirstea. If you're looking for minute-by-minute updates on the Great British Bake Off, you've come to the wrong place. There will be no baking innuendos here.
Post update
What a difference a year makes. Twelve months ago Heather Watson was dreading playing at the US Open. She was low on confidence, low on preparation and low in the rankings. But those tennis blues have melted away and the Briton is ready to make a brand new start of it, in old New York.