British number two Dan Evans was the first to fall. He was hit with a point penalty for swearing as he crashed out to American qualifier Ernesto Escobedo.
Evans was penalised midway through the deciding set as he slipped to a disappointing 7-5 0-6 6-3 defeat.
For the 20-year-old Escobedo, victory over Evans was his first against a top 50-ranked opponent and is likely to see him crack the top 100 for the first time.
Kyle Edmund would have usurped Evans as British number two had he beaten American qualifier Jared Donaldson but instead he blew a one-set lead as he crashed to defeat.
Edmund broke twice to win the opening set 6-2 but Donaldson rallied to win the second-set tie-break to level the match before securing a 2-6 7-6 (7/4) 6-2 victory.
Aljaz Bedene's winning streak also came to an abrupt end as he was forced to retire midway through his first-round match against Germany's Jan-Lennard Struff.
The British number four was playing his seventh match in as many days after winning a Challenger title in Irving last week - and his exertions told as he called it a day when trailing 7-5 4-0.
In the women's draw, Heather Watson failed to repeat her heroics at the same tournament last year as she fell to a costly defeat to Romanian qualifier Patricia Maria Tig.
Watson's 7-6 (7/4) 6-1 loss means she will drop the ranking points she accrued on her run to the fourth round 12 months ago, and is set to slip further away from the top 100.
Securing a place in the main draw of the French Open now looks highly unlikely.
Defeats for the quartet mean Johanna Konta is the only Briton left in the draws.
She will open her campaign against qualifier Aliaksandra Sasnovich on Friday.
Elsewhere at Crandon Park, wins for Kevin Anderson, Viktor Troicki and Dudi Sela saw them set up second-round meetings with Kei Nishikori, Milos Raonic and Rafael Nadal respectively.
In the women's event, Olympic champion Monica Puig crashed out, suffering a 6-2 6-4 defeat to Sorana Cirstea.
