Andy Murray’s struggle to rediscover his form continued on Thursday with a straight-sets defeat to Borna Coric at the Madrid Open.

The young Croat had defeated the world No1 easily at the Dubai Championships in early 2015 and he did so again in the Spanish capital where Murray made 55 unforced errors and lost 6-3, 6-3. It is another defeat which will hurt the Scot, who has failed to reach the quarter-finals in any of his last three Masters series events.

Murray’s form is in stark contrast to that of this time last year when, as the No2 in the rankings behind Novak Djokovic, he embarked on a phenomenal seven-month run to propel himself to the top of the pile. He may now have to do something similar to stay there but it is clear he is still finding his way after an elbow problem ruled him out in the early spring. Cutting out the unforced errors would help.

The signs were there early on in Thursday’s match, particularly on the forehand side, that perhaps Coric could cause Murray problems and he managed a break of serve to lead 3-2 in the first set. However, a couple of forehand mistakes from Coric allowed Murray to break straight back only for the world No1, twice a Madrid winner, to lose his serve once more to fall 4-3 behind.

Coric comfortably held and forced Murray to serve to stay in the set but he could not do it. He made it back to 30–40 and an incorrect call from a line judge gave him another opportunity but the 20-year-old finally broke again to take a scrappy set.

The world No1 was in trouble during his first service game of the second set too, after two more unforced errors helped earn Coric another break point. Murray managed to peg him back before a timely ace rescued the game.

The Scot had two break points of his own in the next game but could not take them and he became increasingly irked at how the match was panning out when it reached a crucial stage at 4-3 to Coric. His frustration boiled over in the eighth game as he lashed a backhand well wide to go 0-40 down on his own serve. Coric then won a long rally to break the No1 and give himself the opportunity to serve for the match.

The world No59 saw it out, handed two match points by another mistake from Murray and winning in just under 90 minutes with a drop shot which his opponent could only lift over the net and wide. The Scot now heads to Rome for next week’s Masters event as the current champion but under distinct pressure. “It feels great, I don’t know how I did it but I played perfect during the game and I’m very happy about it,” Coric told Television Española. “I couldn’t imagine this when I woke up this morning, I always believed but I wasn’t sure how I was going to play but in the end I played as I had dreamed.”

Coric was handed a reprieve despite losing in qualifying to Mikhail Kukushkin and will face Dominic Thiem, who defeated Grigor Dimitrov 4-6, 6-4, 7-6, in Friday’s quarter-finals.

Djokovic, the world No2, reached the last eight with a 6-4, 7-5 victory over Feliciano López. The Serb, who won the tournament last year, had fewer problems against the 35-year-old Spaniard than in his three-set, second-round test with Nicolás Almagro a day earlier, taking the first set before breaking in the 12th game of the second to avoid a tie-break. Rain in the Spanish capital meant the roof on the Manolo Santana court was closed for the match, slowing down López’s powerful serve to the advantage of the Serb.

Djokovic now faces the No8-ranked Kei Nishikori of Japan, who saw off David Ferrer of Spain 6-4, 6-3, and joins David Goffin in the quarter-finals after he beat Canada’s Milos Raonic – who is ranked four places above him – 6-4, 6-2. Belgium’s world No10 next faces Rafael Nadal, who defeated Nick Kyrgios 6-3, 6-1.