Anastasia Potapova stunned seventh-ranked Ons Jabeur 6–4, 7–6 (4) on Wednesday to advance to the quarterfinals of the Cymbiotika San Diego Open.
It was the fourth time this year the 27th-ranked Potapova beat a player ranked in the top 10.
"I think it was my patience, just to stay focused as much as I could because Ons is very tough to play," the 22-year-old Russian player said.
"She's truly one of the best in the world. With her style, it's very uncomfortable for myself, but I'm really happy and proud to get this win."
Jabeur, from Tunisia, was the top seed. She landed just 39 of 85 first-serves, had nine double-faults and could never really find any rhythm. But Potapova said it was still difficult to close out the three-time Grand Slam finalist.
"Tell me about how popular she is – my mother, she loves her so much," Potapova said.
"Every time I talk to my mom on FaceTime with Ons around, she's like, 'Call Ons! She's my best player!' They're actually pretty good friends. I don't know if she's happy that I won today."
No. 3 seed Maria Sakkari of Greece advanced when Colombian qualifier Camila Osorio bowed out due to a right thigh injury, winning 6–3, 2–2.
"It's always very sad and unfortunate to see any athlete, any player, being injured and not being able to compete at 100 percent," Sakkari said.
"She was playing great in that first set. I wish her a speedy recovery."
Second-seeded Caroline Garcia avenged a three-set loss to Sloane Stephens last month in Cincinnati, advancing to the quarters via a 6–3, 3–6, 6–1 win. In the process, the 10th-ranked Garcia improved her head-to-head advantage to 5–3.
"Today was a revenge day," said Garcia, who converted four of her seven break-point opportunities.
"The beginning of the third set was really tight. I faced a break point in the first game and managed to stay in front. That was super important. The last couple of months wasn't always great tennis, but we try to fight, we try to train hard."
Czech Barbora Krejcikova, the fourth seed, overcame eight double faults to oust Ukraine's Anhelina Kalinina, 6–3, 6–2. The wild card snapped a post-Wimbledon skid that saw her go 0–3 without winning a set.
"It was really a difficult first set, even though the score doesn't look like that. I wouldn't really say I was in control after that, but I was just trying to hit my shots and stay aggressive," said the 2021 French Open winner.
American wild cards Sofa Kenin and Katie Volynets faced off at the same site where each won a USTA Girls' 18s National Championship, with Kenin rallying to win 1–6, 6–4, 6–2.
American Danielle Collins was a 2–6, 6–3, 6–4 winner over Latvia's Jelena Ostapenko, while former University of Virginia standout and 2021 NCAA singles titlist Emma Navarro beat Aliaksandra Sasnovich, 7–5, 6–4.