Danish fourth seed Holger Rune crashed out of the US Open on Monday, losing in four sets to Spanish journeyman Roberto Carballes Baena.
Rune, beaten in the quarter-finals of Wimbledon by Carlos Alcaraz, succumbed 6-3, 4-6, 6-3, 6-2 in 2hr 42min.
Carballes Baena, a clay-court specialist ranked 63rd in the world who has never been further than the third round at a Grand Slam tournament, will face either Russia's Aslan Karatsev or the Czech Republic's Jiri Lehecka in the second round.
The defeat was the latest in a series of disappointing early exits for Rune, the 20-year-old regarded as one of the brightest young talents in men's tennis.
Since his quarter-final loss to Alcaraz at Wimbledon, Rune has suffered first round losses at the Canadian Open, the Cincinnati Open and now the US Open.
The young Dane, who reached the third round of the US Open last year, had lashed out at organisers before this tournament, complaining about the decision to place his opening game on Court No.5 of the USTA's Billie Jean King National Tennis Center.
However Rune refused to blame the choice of court for his defeat on Monday.
"It's not difficult, but it's different," Rune said. "You're more close to all the people. It's a nice atmosphere. It's no problem with the court.
"That's obviously disappointing, but not going to blame the court on the loss. He played very, very solid. Credit to him."
Against Carballes Baena on Monday, Rune was always struggling to find his rhythm, racking up 43 unforced errors and seeing his serve broken seven times by the 30-year-old Spaniard.
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