Heather Watson
Heather Watson loses in three sets in the first round of the US Open. [Reuters]

Heather Watson was unable to hold back the tears after she failed to make the most of a superb start as she fell in three sets to 21st seed Simona Halep in the first round of the US Open.

The 21 year old has struggled to find her best form since suffering from glandular fever earlier in the year but started well and looked as though she would be able to cause a major upset.

Watson claimed the first set in impressive fashion and could have moved 5-4 up in the second had she managed to hold her serve. However, she lost four points from being 40-0 up and then double faulted to hand Halep the all important break.

After losing the second set she was unable to make the breakthrough in the decider and eventually went down 4-6 6-6 6-2.

"I thought I played better than I have been. I was playing one of the hottest players on tour right now. She's playing very well. She made a lot of balls," said Watson.

"I thought I played a good two sets and in the third set my fitness let me down a bit. I was cramping in my legs. I think partly because in the first two sets I was a bit tight.

"All in all I'm pleased with how I played but I had my chances definitely and I had an opportunity to win that match in two sets. But she's got the confidence and she won the points when it mattered.

"I think that's why it hurts a lot is because I was so close. But if I keep making opportunities for myself I'll take some."

Watson was struggling towards the end of the match as she looked completely drained and she is unsure whether she has fully recovered from the bout of glandular fever which kept her out of action for two months between March and May.

"I've been doing a lot. I've been running about four times a week, I've been in the gym every day doing weights. I don't quite know what it is," added Watson.

"It could be a few things - just getting back to match fitness because when you're practising you play free and loose but in a match situation there's a lot more pressure."