A "serious competitor" refers to someone who is strong, skilled, and determined in a particular activity or competition. They are likely to pose a significant challenge and cannot be underestimated. Full definition
Why do you think it takes serious competitors in every dog sport, or guide dog trainers, or police dog trainers at least 2 years before they enter their dogs into service? (samthedogtrainer.com)
Until recently digital publications and magazines in particular were not serious competitors to their print counterparts. (turn-page.com)
These international schools are becoming very serious competitors and they are doing it very quickly. (fortune.com)