Davis holds without too much bother there... It feels as if she is gearing up for another attack on the Watson serve. The Briton has been pushed well outside her comfort zone by the world number 84 today, and she's looking a bit frustrated.

Grigor Dimitrov has seen off Australia's Matthew Ebden 6-4 6-2 6-4, while women's 15th seed Aga Radwanska has beaten Katerina Siniakova 6-2 6-3.

Watson punches a confident volley and then forces Davis to net a volley for a 30-0 lead. The American nets again from the Briton's next serve, before a smart return gets Davis on the board. Watson needs something good here. She brings out a massive serve that Davis cannot get to. A hold to 15, and the second set is level.

Joel Newnham:  At this rate @johannakonta could be British number 1

Matthew Vernon:  Good to see a raucous crowd at Flushing Meadows...not. Nothing like the enthusiasm of Wimbledon.

But as the finish line approaches, now it's Davis who gets edgy, handing Watson two break points. The first is saved with an ace. At 30-40, Davis first serves into the net; Watson reaches the second serve and the American can't handle the pressure, looping a forehand long. A quick break back for Watson... who has to hold her own serve now to build some momentum.

Image source, Getty Images

So it's Watson on serve, fighting to avoid opening-day elimination on Court 17 at Flushing Meadows. And she's in trouble; a long backhand gives Davis a 15-30 lead. The Briton bounces on her toes, trying to figure out a way past her opponent, but a wide backhand gives Davis two break points. Watson's first serve is into the net; her second is short - Davis attacks, and the Briton nets with a weak forehand. 

It's Davis's match for the taking now.

Davis eases into a 30-0 lead, before Watson shows some of that fight with a fierce cross-court winner. The American goes to 40-15, but then nets a volley - she looks a little edgy herself. However, Watson volleys a forehand into the net, and Davis holds. The 21-year-old is showing plenty of power here.

It's been a poor 2015 so far for Grigor Dimitrov but the 17th seed has made a good start to his US Open campaign. The Bulgarian leads Matthew Ebden 6-4 6-2 2-3, although the Australian has a break in the third set.

Not the most convincing service game from Watson, and certainly nowhere near as imposing as she was on serve in the early stages. But she holds. She needs all her fight here.

British Davis Cup player Colin Fleming:  "I would try to encourage Heather to relax a little bit. She's got a lot of muscle tension. When that happens, you lose a little bit of pace in your shots. That's allowing Davis to step in and take control. I would also tell her to test the Davis forehand more."

There's full commentary here on the BBC Sport website. Click the 'live coverage' icon at the top of the page.

Watson was serving well early on, but it all went a bit pear-shaped as the first set progressed. Davis, who was also looked edgy as she got close to the finishing line, holds without too much trouble in the opening game of the second. Watson has a lot of work to do against the 5ft 2in American, who is proving a very awkward customer.

Piers Newbery
BBC Sport at Flushing Meadows

It's a sticky 28C and rising on court 17, and Watson is in for a tough afternoon if she is to come through this. Former singles finalist Greg Rusedski is courtside, a man who knew a thing or two about serving and has been helping Watson in that department. She could do with some help on her wayward forehand right now.

There are some big names in trouble elsewhere. Last year's beaten men's finalist Kei Nishikori  lost the first set 6-4 to Benoit Paire, but he has hit back to take the second set 6-3. 

On Arthur Ashe Stadium, women's seventh seed Ana Ivanovic  also dropped the opener, 6-4 to 2014 Australian Open finalist Dominika Cibulkova. It's 4-3 to Ivanovic, on serve, in the second set.

Image source, Getty Images

Former US Open doubles champion Andrea Hlavackova on BBC Radio 5 live sports extra:"Lauren feels Heather is not going to attack her, so she feels more comfortable. Heather has to keep on serving well and maybe step up a couple of returns, not just put them into the court, but attack and wrong foot Lauren."

Davis narrowly avoids serving a double fault with a second serve that clips the top of the net... a terrific rally follows, with both players whizzing round the court. Davis' backhand is a big weapon, though, and she draws in Watson before cutting her off with a stylish cross-court winner. 5-2.

Watson hits back on serve for 5-3, but Davis crunches a shot past the Briton on the next point. Is it in? Watson challenges - but it's good. Davis has serve and three set points. The first serve is long; the second is returned well by Watson, but Davis defends well. Watson blinks first with a long forehand... and it's the world number 84 who takes the first set.

Image source, Reuters

Watson wins the opening point on serve, but Davis responds with two aces for a 2-1 lead, then pounces on a second serve with a huge forehand just inside the baseline for a mini-break. Watson holds the next point at the end of a rally, but a long backhand allows Davis to maintain her advantage at the change of ends.

Davis wins the first point, then slips while looking to change direction on the second and ends up doing the splits, which looks painful. The American seems to lose momentum there, losing the next two points to give Watson a chance at 15-40. Davis misses her first serve, and then hands the Briton the game with a backhand error. It's another break - Watson has saved herself again, and we're into a tie-break...

All of a sudden, Watson is in trouble again. She starts with a double fault, and things slip away quickly from there, as Davis gets two break points. The American keeps up the pressure, rushing into the net. Watson sees an opening down the line, but misjudges the backhand, which is wide. It's the third successive break of service - and gives Davis a second chance to serve for the set. Loose game that from Watson.

Michelle:  Strong backhand from @HeatherWatson92, external come on win this match #USOpen2015, external

Image source, Getty Images

Davis starts with a double fault, but quickly recovers for a 30-15 lead. The nerves strike the American, hitting a forehand long to allow Watson back to 30-30 - and the Briton pounces on that tension, with a backhand that Davis cannot return. A break point for Watson leads to a tense rally - a chance! But she misses! A forehand down the line from the British number one is long, and we're at deuce.

Another long Watson forehand presents Davis with a set point. The American can't take the chance, though, netting with a forehand volley. Both players look very jittery here. Now Watson crunches a forehand down the line - what a response! Another break point. And this time the Briton takes her chance, holding her nerve in the rally as Davis dumps the ball into the bottom of the net with a tight forehand volley.

It's a break apiece - we're level again. Watson has, for now, saved the first set.

(* denotes next server)