Russia's Liudmila Samsonova came from a set down to beat Naomi Osaka 3-6, 7-6 (7/3), 6-4 in the first round of the Berlin Open WTA event on Tuesday.
The world number 20, who won the tournament in 2021, took two hours and 36 minutes to get past the four-time Grand Slam winner.
Samsonova's victory sets up a last 16 clash with American world No 3 Jessica Pegula on Wednesday.
"I was just trying to fight until the end," Samsonova said of her struggles early in the match, adding she wanted "to push and try to find my game".
Osaka broke her opponent twice in the opening set but Samsonova held her serve in the second before winning a tie break, winning the final four points.
Both exchanged breaks in the final set before the Russian 26-year-old broke a second time, putting her on track for victory.
Former champ rallies on 💪@LiudaSamsonova comes back from a set down to defeat Osaka in a gritty match up 3-6, 7-6(3), 6-4.#BTO pic.twitter.com/rd5F6BU93t
— wta (@WTA) June 17, 2025
Former world number one Osaka's defeat is the latest setback in her comeback since returning from maternity leave last year, having also lost in the first round of the French Open.
Sixth-ranked Keys also failed to make it past the opening round, losing to 2023 Wimbledon winner Marketa Vondrousova 7-5, 7-6 (8/6).
The victory was Vondrousova's first over a top-10 opponent this year as she battles her way back from a shoulder injury.
"I'm a player who needs matches to play good," Vondrousova. "And the shoulder is good now, I'm very happy to be back, and I'm hoping to play as many matches as I can."
One of the oldest women's tournaments in the world, the Berlin Open switched from clay to grass courts in 2021 and has since become a popular warm-up event for Wimbledon, which starts at the end of the month.
World number four Zheng Qinwen pulled out of the tournament through injury on Tuesday.
She was replaced by lucky loser Ashlyn Krueger, who lost to 2022 Wimbeldon champion Elena Rybakina 6-3, 7-6 (7/3).
The top four seeds at the tournament, including French Open winner Coco Gauff and runner-up Aryna Sabalenka, along with Pegula and Jasmine Paolini, all enter at the last-16 stage on Wednesday.