Enquiry concerns 17 lower-tier tournaments with estimated takings of over half a million euros. Some players were promised 500 euros but paid only 50 euros in the end.
Spanish police arrested 34 people, including low-ranking players, last month from a criminal network that fixed tennis matches in Spain and Portugal, authorities said on Thursday.
All those arrested are Spanish and include six unnamed tennis players whose international rankings are between 800 and 1,400 in the world. Their Spanish rankings ranged between 30 and 300.
Players were offered sums of between 500 euros and 1,000 euros to fix matches, a police spokesperson said.
"Sometimes they promised 500 euros and in the end only paid 50 euros. The players were above all the victims," the spokesperson added.
The investigation, which began in 2013, concerns at least 17 tournaments in five cities, including Madrid, Seville and Porto. In some cases, the players were asked to lose specific points or games.
The match-fixing network is estimated to have made over half a million euros from lower-tier tournaments in Spain and Portugal.
34 arrests in Spain over #MatchFixing at tennis comps in cities like Pontevedra, Huelva, Badajoz. Why is betting allowed at such events?— James Badcock (@jpfbadcock) December 1, 2016
Two alleged leaders of the network were among those arrested across 12 Spanish cities. The suspected leaders were based in Seville and La Coruna.
The latest arrests — for fraud, corruption and belonging to a criminal organisation — took place in November, police said. If convicted of corruption in sports, they could face prison sentences of up to four years.
Police said several Futures and Challenger tournaments were investigated in Iberia for the past several months and evidence was found that results were rigged.
The Challenger tournaments are second-tier events organised by the ATP, while the Futures are single-week competitions organised by the International Tennis Federation offering either around 9,400 euros or 23,500 euros in prize money.
There were nearly 39 Futures tournaments in Spain this season, and more than 10 in Portugal. Spain hosted Challenger events in Sevilla and Segovia.
The investigation began after a tip given by a player to the Tennis Integrity Unit, the sport's anti-corruption body.
Authorities took the case forward after noticing an unusual amount of online bets related to the suspected tournaments.
The network allegedly used instant-messaging groups and social media to attract online betters who would pay for the information about rigged results.
"Investigation of corruption allegations by law enforcement agencies takes precedence over tennis disciplinary action," the body said. "The TIU will continue to work co-operatively with (Spanish police) and offer its full support and access to resources."