Naomi Broady and Dan Evans made it four Britons through to the second round of the US Open - with Andy Murray still to play his first match.
Broady beat compatriot Laura Robson 6-7 (4-7) 6-3 6-4 to claim only the second Grand Slam main-draw win of her career.
Evans earlier saw off American Rajeev Ram 6-2 4-6 7-5 6-1 to join Kyle Edmund and Johanna Konta in the last 64.
Compatriots Heather Watson and Aljaz Bedene both suffered first-round defeats at Flushing Meadows.
Watson struggled with illness as she lost 6-2 7-5 to Dutch qualifier Richel Hogenkamp, and has now failed to reach the second round at Flushing Meadows in six attempts.
Bedene was beaten 6-4 6-4 6-4 by Australian Nick Kyrgios, with Murray taking on Czech Lukas Rosol later on day two.
I was struggling to breathe - Watson
British number two Watson was in tears after losing the first set in 32 minutes, before being massaged by a trainer and given pills by a doctor.
"It was horrible to play. I was very ill today," she said. "I've had a fever for the last three days but playing in this heat is almost impossible when you feel that bad.
"I was struggling to breathe and then my back, I don't really know what it is, but I've been to see the doctor.
"I'm going to get some blood tests done and figure out what it was."
Asked if the symptoms resembled her debilitating bout of glandular fever in 2013, Watson added: "I feel similar, yeah."
Broady wins tense battle of Brits
There was a lot on the line for both women in the all-British clash on court 13, and it was Broady who proved the stronger at the end of a tight contest.
The 26-year-old from Stockport, ranked 82nd, fought back from a set down to claim her second victory at a major - the other coming at Wimbledon two years
Broady served superbly under pressure, saving eight of nine break points and firing down 16 aces.
She goes on to face fourth seed Agnieszka Radwanska of Poland or American Jessica Pegula in round two.
Leon Smith, GB Davis Cup captain, on 5 live sports extra: "It was a good match - it's always tough when it's two Brits playing each other.
"Naomi is probably more match tight and confident, Laura is still getting the belief back.
"But it's good to have another Brit into the second round of a main draw, even if one has to go out. It's a great thrill having Brits getting through, it gives everyone momentum and plenty to talk about."
Pressure on Zverev in round two - Evans
Evans continued the form that saw him rise to a career-high ranking of 60 earlier this month as he beat Ram in four sets.
"It was good to come through, I enjoyed it out there and started to play a bit better towards the end," Evans told BBC 5 live.
The 26-year-old, who reached the third round in 2013, next faces Germany's Alexander Zverev, the 19-year-old touted by many as a future world number one.
"He's younger than me but I'd say he's more experienced on the ATP Tour, so I'd say I don't have much to lose," said Evans.