Ons Jabeur said an illness has left her feeling like a "zombie" but the fifth seed showed will determination to battle past Czech teenager Linda Noskova 7-6, 4-6, 6-3 to reach the US Open third round.
Tunisian Jabeur suffered breathing difficulties during her gritty first-round win over Colombia's Camila Osorio and was not at her best against Noskova, getting through a tough match in a little over two hours.
Jabeur, 29, said she had been struggling for about a week with the illness and was "taking a lot of medicine".
"I'm a zombie because I have a flu," said Jabeur. "I'm doing everything I can with my team trying to recover," she added. "They have amazing doctors here, so they've been helping me. I basically took every medication they have.
"I'm glad it was two matches, two tests. I was trying to push myself to see what I can do, getting out of the comfort zone. Hopefully I'll continue in better shape and feel better for the next match."
Jabeur faces another Czech in Marie Bouzkova and is determined to continue her run.
"Emotionally I could be tired, but I know if I just let go I'll regret it after, so I want to continue and stay in New York as long as I can," she said.
Several players at the year's final Grand Slam have been hit by illnesses, including Dominic Thiem, Chris Eubanks and Hubert Hurkacz.
Carlos Alcaraz and Daniil Medvedev moved quietly into the third round while big-hitting American John Isner delivered his trademark stack of aces in taking his final bow at Flushing Meadows.
Alcaraz entertained a record crowd at Arthur Ashe Stadium with a 6-3, 6-1, 7-6 win over Lloyd Harris but the match also revealed a lack of sharpness which the Spaniard will need to rectify if he is to successfully defend his US Open crown.
Some dazzling shot making from the world No 1 had the crowd on its feet but Alcaraz also piled up 29 unforced errors and showed a lack of concentration at key moments.
The 20-year-old will need to improve with rival Novak Djokovic on the prowl for a 24th Grand Slam and seeking revenge for his Wimbledon loss to the Spaniard.
Medvedev survived a late night fright before taming Australian battler Christopher O'Connell 6-2, 6-2, 6-7, 6-2.
After strolling to a two set-lead Medvedev had looked headed for routine passage but in the end the third seeded Russian needed all his resolve to push O'Connell to the exit.
Isner, the standard bearer for American men's tennis during some barren years, never made it past the quarter-finals in 17 US Opens but his towering presence and booming serve made him a crowd favourite.
He signed off his professional singles career with a 3-6, 4-6, 7-6, 6-4 7-6 loss to fellow American Michael Mmoh but did not go quietly, adding 48 more aces to his record total.