Madrid - Rafael Nadal suffered his first clay-court defeat in almost a year when the world No 1 was shocked 7-5, 6-3 by Austria's Dominic Thiem in the Madrid Masters quarter-finals on Friday.
Defending champion Nadal, who came into the tournament having won both Monte Carlo and Barcelona for an 11th time, also saw his record 50-set winning streak on clay ended by the fifth-seeded Thiem.
Thiem, the runner-up to the great Spaniard in the 2017 Madrid final, had been the last player to defeat Nadal on clay at the Italian Open in Rome almost 12 months ago.
The shock result on Friday also means that Roger Federer will reclaim the world number one ranking on Monday.
"I wasn't good enough with my forehand or my backhand to open to his forehand today," admitted Nadal.
Five-time Madrid winner Nadal, who will be bidding for an 11th French Open title in Paris later this month, had won his previous 21 matches on clay.
That run included him surpassing John McEnroe's 1984 record of 49 sets won in succession on the same surface during his last-16 duel with Diego Schwartzman on Thursday.
However, in their ninth career clash, Thiem put Nadal under pressure almost from the start on Friday and broke for a 4-3 lead in the first set.
A double fault and a long forehand allowed Nadal to retrieve the break at 5-5.
It was a brief reprieve as a lazy forehand volley gave Thiem a 6-5 advantage and the set came the Austrian's way in the 12th game off an ace.
Thiem, watched by his girlfriend - and WTA star - Kristina Mladenovic, turned the screw for a 3-1 lead in the second set but Nadal battled back to 3-3.
That was as good as it got for the Spaniard as Thiem broke again for 4-3, held to love for 5-3 and secured victory with another blistering forehand.
Thiem, who next faces South Africa's Kevin Anderson for a place in Sunday's final, joins Novak Djokovic and Gaston Gaudio as just the third man to have beaten Nadal three times on clay.
World No 8 Anderson battled into his first Masters semi-final with a 7-6 (7/3), 3-6, 6-3 win over Serbian qualifier Dusan Lajovic.
Anderson, the 31-year-old US Open runner-up last year, hit 15 aces and 35 winners.
Dutchwoman Kiki Bertens continued her remarkable run at the Madrid Open by thrashing French seventh seed Caroline Garcia to reach the womens' final.
The world No 20 will face either Petra Kvitova or Karolina Pliskova in the biggest match of her career on Saturday, after brushing aside Garcia 6-2, 6-2.
Bertens, who had already stunned world number two Caroline Wozniacki and five-time Grand Slam champion Maria Sharapova earlier in the week, started brightly and broke twice en route to the opening set.
Garcia struggled throughout and never even mustered a break-point opportunity, falling in the semi-finals for the second straight tournament after losing to CoCo Vandeweghe in Stuttgart.
Bertens, whose five previous WTA titles have all come on clay, was rock-solid in the second set as she wrapped up the win in only an hour and seven minutes.
Two-time Wimbledon champion Kvitova takes on her fellow Czech Pliskova in the second last-four clash.
Results from the ATP/WTA Madrid Open on Friday (x denotes seeding):
Dominic Thiem (AUT x5) bt Rafael Nadal (ESP x1) 7-5, 6-3
Kevin Anderson (RSA x6) bt Dusan Lajovic (SRB) 7-6 (7/3), 3-6, 6-3
Kiki Bertens (NED) bt Caroline Garcia (FRA x7) 6-2, 6-2