Wimbledon champion Garbine Muguruza suffered a 6-2, 6-4 defeat at the hands of unseeded Czech Barbora Strycova in the second round of the Nature Valley Classic in Birmingham on Thursday.
The experienced Strycova, a runner-up at this Edgbaston tournament in 2014 and 2016, picked up a double break early in the first set and eventually defeated the top seed with surprising ease in just under one and a half hours.
The defeat comes as a setback for the Spaniard's grasscourt preparations ahead of Wimbledon, which starts on July 2.
Defending Birmingham champion Petra Kvitova continued her impressive grasscourt form after overpowering Australia's Daria Gavrilova 6-2, 6-2 to book a spot in the quarter-finals.
Fourth seed Kvitova broke early in the first set and hit some powerful backhands and strong returns to wrap up the contest in one hour and 18 minutes.
"I think a few of the games were very tight, especially when you need to break. It's always tough to get the break on the grass," Kvitova, a two-time Wimbledon champion, said.
"I think there's still a few things I can improve -- the serve, probably. You need to serve well on the grass.
"The conditions were pretty tough today with the wind."
The 24-year-old will face either Germany's Julia Goerges or Australia's Ashleigh Barty in the quarter-finals on Friday.
Federer saves two match points in Halle win over Paire
Defending champion Roger Federer saved two match points in a final set tiebreak before defeating Benoit Paire 6-3, 3-6, 7-6(7) in a thrilling contest to reach the Halle Open quarter-finals on Thursday.
Federer, who must win a record-extending 10th title in Halle to remain at the top of the world rankings, dropped a set for the first time in six meetings against the erratic Frenchman as the pair fought out a gripping two-hour duel.
The 36-year-old Swiss extended his winning run on grass to 18 matches as he continues a serene build-up to his title defence at Wimbledon next month but after easing through the first set found Paire in one of his more inspired moods.
Australian Matthew Ebden, competing in Halle at a career high ranking of 60, rallied from a set down to beat local hope Philipp Kohlschreiber 4-6, 6-1, 6-2 and will face Federer for the first time in his career in the last eight.
Croatia's Borna Coric, who defeated second seed Alexander Zverev in the opening round, continued his steady progress with a 6-4, 6-2 win over Georgian lucky loser Nikoloz Basilashvili.
The 21-year-old won 81 percent of his first serves and struck 20 winners in a dominant performance to book a place in the quarter-finals.
US Open rolls in new changes for ballpersons
Ballpersons at the US Open will now roll the ball from point-to-point, rather than throw between positions, the United States Tennis Association said on Thursday in a bid to grow the number of applicants.
The USTA said rolling the ball between positions, a style that has traditionally been utilized at the other grand slam events, puts less emphasis on a single skill-set -- in this case throwing -- and more focus on speed, dexterity and agility.
"In the past, potential applicants may have decided not to try out because of the daunting nature of the throw," Stacey Allaster, the USTA's chief executive for professional tennis, said in a statement.
"We hope that those same individuals will now come out for the try-out this year, as well as those that may have not made the cut in the past, solely due to their throwing prowess."
The changes take effect beginning with this year's U.S. Open, which will be held from Aug. 27-Sept. 9 in New York City.
More than 100 US Open ballperson positions will be up for grabs when public tryouts are held on June 26 at the USTA Billy Jean King Center.