Seeds David Ferrer, Taylor Dent and Fernando Verdasco suffered surprise first-round defeats at the Stockholm Open on Monday.
Spanish second seed Ferrer, who is in contention to qualify for the Masters Cup in Shanghai, failed to sustain a strong start and lost to compatriot Alberto Martin 2-6, 6-3, 6-2.
Having easily clinched the first set and broken Martin in the second, he looked on his way to a comfortable win only for his game to crumble after his opponent broke back to 3-3.
Martin grabbed the following five games to take the set 6-3 and a 2-0 lead in the decider.
Ferrer, whose frustration begun to show in the second set when he threw his racket down several times, found no answer to Martin's aggressive tennis in the third set and was broken again to trail 4-1.
The Spanish claycourt specialist went on to serve out the set for his third win in seven
Big-serving American Dent was beaten 2-6 6-4 6-4 by Italian veteran Davide Sanguinetti, a semi-finalist here in 2003.
Thailand's Paradorn Srichaphan, the 2002 winner, beat eighth-seeded Spaniard Fernando Verdasco 7-6, 6-3.
In a close match the Thai raised his game at just the right moment, breaking the Spaniard twice from 3-3 in the second set while conceding just three points to clinch the match.
Dent and Verdasco's losses mean top seed and defending champion Thomas Johansson cannot meet another seed until the semi-finals.
Johansson, who defeated Andre Agassi in the final last year, faces fellow Swede Ervin Eleskovic, a wild card, on Tuesday.
American Vincent Spadea overcame Argentine Juan Ignacio Chela 6-2, 4-6, 6-1 to set up a possible second-round meeting with Johansson.