Cristian Garin was on the end of a shock thrashing by Denmark's Holger Rune in the first round of the Serbia Open on Monday.

The Chilean fifth seed, who was the highest-ranked player in action, is regarded as one of the ATP Tour's best on clay but hardly got a look-in as 18-year-old Rune emerged with an impressive 6-3 6-1 win.

Rune has risen to a career-high 72 in the ATP rankings and showed every sign that he will continue to climb over the season as he produced an aggressive performance that saw him take six of 17 break points.

Playing in only his fifth ATP Tour match on clay, Rune needed just an hour and 25 minutes to see off Garin – the winner of five tour-level titles on the surface – and set up a second-round clash with either Dusan Lajovic or Taro Daniel.

Sixth and seventh seeds Fabio Fognini and Miomir Kecmanovic avoided similar shocks, though their respective wins were wildly different.

Kecmanovic crushed veteran Richard Gasquet 6-0 6-3, while Fognini was forced to overturn a one-set deficit in his defeat of fellow Italian Marco Cecchinato 6-7 (5-7) 6-2 6-2.

No seeds were in action at the Barcelona Open, but Italian prospect Lorenzo Musetti enjoyed a hard-fought 7-5 7-5 defeat of Argentina's Sebastian Baez to seal his spot in the second round.

The 20-year-old is now 5-2 for the clay season and will go up against Dan Evans next. Joining Musetti in progressing is another promising youngster in Brandon Nakashima, who beat Nicolas Alvarez Varona 7-6 (8-6) 6-3 to tee up a clash with number two seed Casper Ruud.

Mackenzie McDonald, Elias Ymer, Kwon Soon-woo and Federico Coria were among the other victors, while retiring 2004 champion Tommy Robredo bowed out with a 6-1 6-1 loss to Bernabe Zapata Miralles.