World number three Coco Gauff was pushed hard by American compatriot Sachia Vickery before overcoming the qualifier 6-3, 4-6, 7-5 at the WTA Stuttgart clay-court tournament on Wednesday.
The US Open champion needed two hours 27 minutes to silence her second-round opponent, ranked 134th in the world.
Gauff began brightly, rattling off the first three games, but from then on it was to become an error-strewn struggle.
She double-faulted 15 times, enabling Vickery to get 19 break points, of which she converted seven.
Gauff converted six with the most important one coming in the seventh game of the deciding set.
Trailing 0-2, Vickery fought her way back to take a 4-2 lead, with Gauff then clinching the crucial game on her fourth break point.
“She gets a lot of balls back and I was just trying to be patient and also be aggressive. I think I got passive. She played well and I think I did a good job of staying in the match,” said Gauff.
Gauff will face the winner of the match between Zheng Qinwen and Marta Kostyuk in the quarter-finals.
Former US Open champion Emma Raducanu defeated three-time Grand Slam winner and former world number one Angelique Kerber 6-2, 6-1 in her first-round match.
At the weekend Raducanu led Britain past France and into the finals of the Billie Jean King Cup in November and she continued that clay-court form in Germany.
Raducanu hit 26 winners to 11 unforced errors while two-time Stuttgart champion Kerber was undone by 29 unforced errors.
The British player will next face the Czech Republic’s Linda Noskova where a win would put her into her second Stuttgart quarter-final.
Ninth-ranked Ons Jabeur snapped a five-match losing streak by beating Russia’s Ekaterina Alexandrova 2-6, 6-3, 7-6 (7/1) in the second round.
“I have thought about withdrawing from this tournament so many times because I couldn’t take another loss,” said Jabeur.
“It was very, very difficult.”
The victory over 16th-ranked Alexandrova was just Jabeur’s third win of 2024 and first since beating Raducanu in Abu Dhabi.
Australian Open champion Aryna Sabalenka, runner-up in the German city for the last three years, advanced when close friend Paula Badosa retired from their second round clash with a left thigh injury in the final set.
Sabalenka has taken the first set 7-6 (7/4) but dropped the second, 6-4.
The Belarusian was a break down at 3-1 in the decider before clawing her way back to 3-3 when her Spanish opponent retired.
Sabalenka will face either Wimbledon champion Marketa Vondrousova or Anastasia Potapova in the quarter-finals.