Coco Gauff withdrew during Tuesday's first-round match against the Czech Republic's Marie Bouzkova due to a left ankle injury sustained during the opening set of Bouzkova's 7-5, 1-0 win in the Western Southern Open at Mason, Ohio.
The 11th-seeded Gauff was leading 5-3 in the first set when she tumbled to the court and injured her lower leg at the Cincinnati-area tournament. She lost the next three games before being examined by a trainer.
Bouzkova then finished off the set and won the opening game of the second set before Gauff called it a day.
It was a difficult development for the 18-year-old, who lost in the quarterfinals in each of her previous two events. She also reached the French Open final in the spring before losing to Poland's Iga Swiatek, the top-ranked player in the world.
Gauff is playing doubles with Jessica Pegula in the tournament and there was no immediate word on whether she will continue. There also is the matter of the U.S. Open being less than two weeks away.
Bouzkova faces Alison Riske-Amritraj in the second round.
Meanwhile, Japan's Naomi Osaka also went down in the first round as China's Zhang Shuai outplayed her and won 6-4, 7-5 in just 75 minutes.
Osaka held a 10-4 edge in aces but won just 37.1 percent of her second-serve points in the disappointing setback. Osaka committed 29 unforced errors and had 21 winners.
Zhang, who has four aces, will face Russian Ekaterina Alexandrova in the second round.
Romania's Sorana Cirstea also registered an upset, beating 12th-seeded Swiss Belinda Bencic 6-2, 6-7 (3), 6-4. Cirstea had eight aces and saved five of seven break points during a match that took two hours and 38 minutes.
Czech Karolina Pliskova, seeded 14th, recorded a 7-5, 6-1 victory over Venus Williams. Pliskova had an 8-2 edge in aces.
"I think I finally got it to a better level," Pliskova said. "Hard court is my favorite. I always do well in America, so hoping for more matches here."
Pliskova will next face Belgium's Elise Mertens, who notched a 6-1, 6-1 win over Ukraine's Anhelina Kalinina.
Also, Shelby Rogers hammered out nine aces while thrashing fellow American Sofia Kenin 6-1, 6-2 in just 66 minutes.
In other matches, Russia's Anna Kalinskaya beat Italy's Martina Trevisan 7-6 (2), 7-5, and Ukraine's Marta Kostyuk outlasted Italy's Camila Giorgi 6-4, 5-7, 6-4.
--Field Level Media