The Briton double-faulted 10 times as she struggled with her serve but was still able to see off her American opponent 7-5 6-3 in Acapulco.
Watson, 24, plays second seed Kristina Mladenovic of France in round two.
Fellow Briton Kyle Edmund plays Sam Querrey of the US in the men's draw later on Tuesday.

Zakariya Ashiq, 20, from Coventry, was found guilty of two counts of preparing acts of terrorism at the Old Bailey on Tuesday.
He was arrested at Heathrow Airport in November after failing to cross into Syria from Jordan.
He admitted trying to get to Syria but said he had been forced to leave the UK because he was being "harassed" by MI5.

But did you ever stop to think what this means? What are terms and conditions, and why do you have to agree to them?
Read on to find out more.
Terms and conditions are rules you have to agree to before you can use certain products or services, like social media.
Last year, Newsround found out that many of you have signed up to have social media accounts.
The companies that run these networks - for example, Facebook, Instagram, Snapchat or WhatsApp - all have their own terms and conditions that you have to agree to before you can have an account with them.
For example, they might say that you have to be a certain age to have an account, or that the company is allowed to use any pictures you post however they like.
You might hear of terms and conditions being referred to as Ts and Cs.
You aren't allowed to use these sites without agreeing to the terms and conditions.
So many people - including adults - don't read them because they just want to get on and use the site or app.
They don't think that there's any point wasting time to go through them, which means they don't know what they've agreed to.
Also, the list of rules can be extremely long and complicated - and written in technical language - which can make them difficult to understand.
This means that many people may not know what they've actually agreed to.
It is important to make sure you understand what you are agreeing to so that you know exactly what you've signed up for.
You don't want to get a nasty shock later because you didn't fully understand something that you'd agreed to a while ago.
Jenny Afia, a lawyer who knows all about terms and conditions, says that to make sure you don't get caught out: "Get help from parents or teachers to understand what the terms and conditions mean."
"It's important to be aware of what you're signing up to."

The 19-year-old, who joined from Monaco on transfer deadline day, scored twice in Sunday's 3-2 win over Southampton as United moved to second in the table.
The Frenchman now has three goals from his first two Premier League games.
"He's been absolutely magnificent. They could not have hoped for any more," former Arsenal and England defender Keown told Match of the Day 3.
"There was a huge transfer fee on his head, but it doesn't seem to bother him. Every time he is in front of goal he seems to be at his happiest."
Martial became the most expensive teenager ever when he moved to Old Trafford, and the signing was met with surprise by some commentators in the media.
Already an established France Under-21 international, he had just been called up to the senior national team when he joined Louis van Gaal's side at the beginning of September.
And with team-mate Wayne Rooney having yet to score in the league this season, Martial has already proved vital to United.
"I thought they would take him off at 3-1 and they do need to manage him well. He's only a young boy really," said Keown.
"But he looks very powerful and mature and he's well beyond the age he played at on Sunday."
Heather Watson moved into the second round of the Abierto Mexicano Telcel with a straight sets win over Bethanie Mattek-Sands.
[NEXT_CONCEPT]
A British man who attempted to join Islamic State in Syria has been jailed for six years.
[NEXT_CONCEPT]
When you sign up to have a social media account, you may have to tick a box saying that you agree to the terms and conditions.
[NEXT_CONCEPT]
Manchester United forward Anthony Martial has been unfazed by his "huge" £36m transfer fee, says Martin Keown.