Emma Raducanu outlasted Camila Osorio 7-6(5) 7-6(4) to reach the quarter-finals of the Citi Open in Washington on Thursday night.
"I think I died about three times in that match," Raducanu said afterwards.
"Honestly I don't know how I came back. Camila is such a battler. Every point I was working so so hard.
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"Sometimes it wasn't the strongest shots but you've got to generate over and over again and she's such a fighter.
"Even when I was up in the second set and 4-2, 40-0 and three break points she came back fighting. The dynamic of the match changes. I'm just really pleased to have got through that one."
Raducanu had beaten Louis Chirico in straight sets on Wednesday to reach the round of 16 round of the tournament as she prepared for the 2022 US Open.
Raducanu, 19, admitted after that win that she was still a little rusty after a few months of play interrupted by underwhelming form and injury.
The US Open is the chance for her to defend her famous 2021 title win when she announced herself as one of women’s tennis' next stars.
Despite enduring a difficult period since then as she adjusts to life in the spotlight of the professional circuit, Raducanu has said she does not feel under too much pressure.
She proved she can handle a little stress, at least, at the end of the first set in Washington, when she was taken to a tie-break by her Colombian opponent, Osorio, and won 7-5 to move one set from victory.
Raducanu proved her superiority despite problems with double faults - nine over the course of the match - when she again held her nerve in a tie-break, this time in the second set.
A 7-4 margin was enough to send her through to the third round after two hours and 49 minutes on court.
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