Naomi Osaka was forced to retire from the Birmingham Classic with injury on Wednesday, while Angelique Kerber was surprisingly dumped out of the Mallorca Open.
World number 18 Osaka won her first WTA Tour title at Indian Wells this year, but her hopes of doubling that tally were halted by a fitness issue against qualifier Dalila Jakupovic.
The 20-year-old had only a brief discussion with her trainer before deciding not to continue after losing the first set 6-3.
This was a second big scalp in succession for Jakupovic, who knocked Elise Mertens out in the first round, but her immediate focus was on Osaka's welfare.
"It's never nice to win when somebody's injured," she said. "I was trying to focus on my play and I didn't really look on [Osaka's] side. I hope she is going to be better for the next tournaments."
Tested by Donna Vekic 24 hours earlier, second seed Elina Svitolina had few problems recording a straight-sets victory over Alize Cornet.
Mihaela Buzarnescu will be Svitolina's opponent in the next round after coming from a set down to beat Petra Martic 4-6 6-3 6-2.
In Mallorca, meanwhile, Kerber lost to Alison Riske in a match that stretched over two days, with darkness having previously stopped play on Tuesday.
The players came back out to continue at 5-7 6-2 1-1, but Kerber lost the next five games to become the fifth seed to crash out in the first round.
There were no such issues for top seed Caroline Garcia, however, as she saw off Johanna Larsson 6-4 6-3 to reach the quarter-finals for the third consecutive year.
Sam Stosur also headed through in straight sets.
18. Juli 2018
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|Mourinho´s pre-season fears mount with Sanchez sidelined|
|Alisson thanks Roma fans ahead of rumoured Liverpool switch|
|Pogba must understand his World Cup heroics, says Mourinho|
|Odriozola excited by Lopetegui reunion at Real Madrid|
|Revis announces retirement from NFL after 11 seasons|
|The Open 2018 diary: Rahm´s saving Grace, Rory´s going grey|
|Kawhi Leonard traded to Raptors as DeMar DeRozan heads to Spurs|
|Cantona, Rosberg and other surprise retirements after Warburton hangs up his boots|
|No doubting Thomas as he climbs into yellow with stage win|
|Liverpool´s Oxlade-Chamberlain to miss ´vast majority´ of 2018-19|
|Open Championship by far the greatest - Player|
|Sarri wants new central midfielder and hints at Chelsea stay for Cahill|
|Nantes sign former Manchester United full-back Fabio|
|Marcelino rewarded with one-year Valencia extension|
|Warburton retires: Bale leads social media tributes to Welsh great|
|Mistakes do not define me, says Chelsea boss Sarri|
|Saints and Tigers to honour Horne with Twickenham clash|
|I want to keep Hazard, says Sarri|
|Europe must stop American juggernaut at Carnoustie|
|Joshua planning for heavyweight ´legacy´ – starting with Povetkin|
|Warburton retires: Wales and Lions star´s career in numbers|
|Matching ´ridiculous´ Tiger not on McIlroy´s radar|
|Spain sent me World Cup SOS, claims Benitez|
|Ozil, Gundogan critics should ´shut up´ - Klopp|
|McIlroy plans to play carefree at Carnoustie|
|Pakistan thrash Zimbabwe to secure series win|
|Warburton retires: Gatland salutes Wales and Lions great|
|Sturridge determined to stay at Liverpool|
|Mbappe is an alien – Varane|
|Warburton retires from rugby|
|India to assess Bhuvneshwar as Bumrah makes Test squad|
|Injured Craddock passes $100,000 in donations at Tour de France|
|WATCH: Perez taunts team-mate after audacious training goal|
|Alison Becker tweets ´I´m joining Liverpool´... but all is not as it seems|
|F1 Raceweek: Hamilton chasing down Schumacher - German GP in numbers|
|Notorious Carnoustie holds fond memories for legendary Watson|
|Rooney refused to be ´dead weight´ at Everton|
|The Open 2018: Molinari, Rahm and the in-form contenders at Carnoustie|
|Marsh interested in Australia leadership roles|
|United can challenge City for Premier League title, says Rooney|
|MLB All-Star 2018: Seven things you missed at Midsummer Classic|
|American League win home run-filled MLB All-Star game|
|Dragons lure Sims from Broncos|
|Cassano: Ronaldo like Nadal, Messi like Federer|
|Lakers´ Ball undergoes successful knee surgery|
|´We kill them, Messi or no Messi´ - Pogba´s pre-Argentina World Cup speech revealed|
|Cardinals suspend GM Keim for five weeks after DUI|
|Dalic: I can coach Brazil or Barca but Croatia my favourite|
|Warriors coach Kerr extends contract|
|I´m ready to bust him up - Miller calls out Joshua after New York confrontation|
|CAF Champions League Review: ES Tunis and TP Mazembe set the pace|
|Mixed day for seeds in Newport as Muller progresses|
|Rams sign Cooks to five-year extension|
|Cowboys’ Randy Gregory reinstated by NFL|
|Verdasco survives Swedish Open scare as Paire is picked off in Umag|
17. Juli 2018
|US Open announces record prize fund for 2018|
|Stosur wins battle of former grand slam champions in Gstaad|
|Emery is Arsenal´s best signing, says Pires|
|Brewster commits to Liverpool with long-term deal|
|Panama boss Gomez steps down after World Cup|
|No Chelsea agreement for Golovin, claims CSKA president|
|Root looks right at home as England ease to series victory|
|Brighton sign Mali midfielder Bissouma from Lille|
|Kaka backs Brazil boss Tite after ´excellent´ World Cup|
|Alaphilippe the toast in France as Van Avermaet increases Tour lead|
|Shkupi 0 Rangers 0: Gerrard´s men advance despite dour draw|
|Khazri ends Sunderland nightmare as Saint Etienne swoop|