Karolina Pliskova and Coco Gauff were left to wonder how they failed to reach the Ostrava Open quarter-finals as both squandered dominant positions on Thursday.
World number six Pliskova, already frustrated by second-round exits from the US Open and French Open since tennis returned from lockdown, suffered a 4-6 6-4 6-3 defeat to Russian Veronika Kudermetova, despite leading by a set and a break at one point.
Pliskova also lost to the world number 47 at the Western and Southern Open in August, and this was another blow to add to the list of frustrating results for the Czech in the past two months.
Qualifier Gauff succumbed 1-6 7-5 7-6 (7-2) in her clash with Belarusian Aryna Sabalenka, despite having led by a double break at 5-2 in the deciding set.
The American 16-year-old believes she can use the setback as part of her tennis education, saying she needed to play a "smarter" game.
"I just need to play a little bit smarter on pressure points, to make my opponent play," Gauff said, quoted on the WTA website.
"I feel like I wasn't nervous today - I think I just need to be smarter. Sometimes I have tendencies, when it gets to the end, to rush out of the points when instead I should just put the ball in the court."
Belgian Elise Mertens eased to a 6-4 6-2 win over Czech player Karolina Muchova, while Ons Jabeur beat Jelena Ostapenko 6-4 6-4 and US Open semi-finalist Jennifer Brady came from 5-2 behind in the first set to earn a 7-5 6-2 victory over Russian Daria Kasatkina.