British pair Johanna Konta and Heather Watson both made losing returns to the WTA Tour with straight-set defeats at the Lexington Open in Kentucky.
Konta, the third seed, was beaten 6-4 6-4 by world number 48 Marie Bouzkova.
The Briton needed medical attention early on but, after having her blood pressure and pulse checked and consulting the doctor, was able to return to court.
Watson struggled in her 6-2 6-1 loss at the hands of American Jennifer Brady.
Konta had also lost to her Czech Republic rival in Mexico in March in her final match before the tour was suspended because of coronavirus.
After play was delayed by four hours because of a rain storm and after surviving a break-point scare in the opening game of the match, which was played in extreme humidity, Konta appeared to be struggling with the conditions.
The 29-year-old has suffered similar issues in the past, notably at the 2016 US Open and at the Fed Cup last year. She recovered to win both matches.
After the treatment, she gradually found her rhythm but was broken in the seventh game of the set as Bouzkova, 22, went 4-3 up and held on to take the set.
Konta took an early 2-0 lead in the second set but Bouzkova broke back straight away.
The world number 15 started to struggle on her serve and her opponent eventually took advantage to go 5-4 up and serve out the match.