Madrid - World number one Andy Murray's slump in form continued as he crashed out of in the Madrid Masters third round 6-3, 6-3 to lucky loser Borna Coric on Thursday.
Defending champion Novak Djokovic did ease into the quarter-finals, though, with a 6-4, 7-5 win over Feliciano Lopez.
Coric was only handed a late reprieve to enter the draw in Madrid after losing in qualifying to Mikhail Kukushkin.
However, Murray cut a frustrated figure as he was swept aside by 20-year-old Croatian for the second time in the pair's four meetings.
Coric broke the fragile Murray serve three times in the first set under the roof at the Manolo Santana centre court due to rain in the Spanish capital.
And one poor service game from the Wimbledon champion cost him dear in the second as Coric served out for the match to set up a quarter-final meeting with Grigor Dimitrov or Dominic Thiem.
Djokovic has had his own struggles of late.
The Serb parted ways with his coaching staff of 10 years last week after also suffering early exits at the Australian Open and Monte Carlo Masters.
However, he believes he is making slow but steady progress after seeing off Lopez and a partisan crowd to reach the last eight.
"If you're playing against a Spanish player in Madrid, it's obviously a different game, it's a different feeling," said Djokovic.
"They have a lot of support. You need to kind of hold your nerve, hold your concentration, and use all the necessary energy at the right moments."
Djokovic was dominant on serve throughout, facing only one break point in the opening game of the second set.
And the 12-time Grand Slam champion applied just enough pressure at the right time to break Lopez's serve in the final game of both sets.
Djokovic faces a much tougher test on Friday against world number eight Kei Nishikori, who earlier swept aside David Ferrer 6-4, 6-3.
However, Nishikori has only won two of their previous 13 meetings.
"Novak is someone that I don't have good record (against)," said the Japanese.
"I got to stay really positive and I got to play great tennis to beat him. I try to enjoy my big challenge tomorrow."
Rafael Nadal's quest for a third straight title after wins in Monte Carlo and Barcelona faces a stiff test against Nick Kyrgios in the night session.
David Goffin awaits the winner of that clash after the Belgian beat world number six Milos Raonic 6-4, 6-2.
Twenty-year-old German Alexander Zverev continued his fine form after claiming his second title of the year in Munich last week as he ousted 11th seed Tomas Berdych 6-4, 6-4.
In the women's Madrid Open, third seed Simona Halep remains on course to defend her title as she thrashed American Coco Vandeweghe 6-1, 6-1.
Halep faces Latvian Anastasija Sevastova in Friday's semi-finals after the world number 22 saw off Kiki Bertens 6-3, 6-3.
France's Kristina Mladenovic also reached the last four in straight sets 6-4, 6-4 over Sorana Cristea.
Collated results from the ATP Madrid Masters/WTA Madrid Open on Thursday (x denotes seeding):
Borna Coric (CRO) bt Andy Murray (GBR x1) 6-3, 6-3
Pablo Cuevas (URU) bt Benot Paire (FRA) 7-5, 0-6, 6-1
Alexander Zverev (GER) bt Tomas Berdych (CZE x11) 6-4, 6-4
David Goffin (BEL x9) bt Milos Raonic (CAN x5) 6-4, 6-2
Kei Nishikori (JPN x6) bt David Ferrer (ESP) 6-4, 6-3
Novak Djokovic (SRB x2) bt Feliciano Lopez (ESP) 6-4, 7-5
Rafael Nadal (ESP x4) bt Nick Kyrgios (AUS x16) 6-3, 6-1
Dominic Thiem (AUT x8) bt Grigor Dimitrov (BUL x12) 4-6, 6-4, 7-6 (11/9)
Kristina Mladenovic (FRA x14) bt Sorana Cirstea (ROM) 6-4, 6-4
Simona Halep (ROM x3) bt Coco Vandeweghe (USA) 6-1, 6-1
Anastasija Sevastova (LAT) bt Kiki Bertens (NED) 6-3, 6-3
Svetlana Kuznetsova (RUS x8) bt Eugenie Bouchard (CAN) 6-4, 6-0