British No1 Johanna Konta required medical treatment inside the opening 10 minutes of her return to action at the Lexington Open in Kentucky as she looked to gain fitness ahead of the US Open later this month.
Having held serve in the first game, Konta – the third seed - halted proceedings two points into the second in her match against Maria Bouzkova.
Hunched over her racket, Konta appeared to be breathing heavily and her face was flushed, probably as a consequence of the extreme humidity as well as returning to competitive action after five months.
Her blood pressure and pulse were checked and following a consultation with the doctor, Konta was able to return to the court.
It was later reported that Konta had suffered a heart palpitation.
She battled on to 3-3 before rising Czech star Bouzkova, 22, secured the first break of the match and took the opening set.
Konta, beaten by the same opponent in her last match before lockdown, secured a break early in the second.
But she immediately dropped serve and a second Bouzkova break secured a 6-4 6-4 victory.
Ahead of the match, Konta had touched on the precautions she was taking in the States amid the coronavirus pandemic.
"I arrived in the States on Thursday, I flew from London to Chicago and then we rented a car and drove down from Chicago," she said.
"We are going to keep that car and drive to New York, just to really mitigate how much time I spend in airports and airplanes and be as vigilant as I can be.
"I feel the same as I did at home. That is one of the things that was quite shocking when I arrived. I was very aware this was a global pandemic, but being just at home and seeing the different protocols that are in place at home, I knew that it happened everywhere but I guess it shocked me.
Additional reporting by the Press Association