Tennis has been responsible for more suspicious betting than any other sport, according to a two-year corruption investigation published on Wednesday.

The Independent Review of Integrity in Tennis claimed the lower levels of the sport provide a "fertile breeding ground" for breaches of integrity and are engulfed in a "tsunami" of betting-related corruption.

The problems stem from too many players in the lower reaches, such as the Futures and Challenger circuits, not earning enough to make a living, coupled with the rise of online betting.

Adam Lewis QC, who chaired the panel, said: "Player-incentive structure and remuneration creates a lamentably fertile breeding ground for breaches of integrity.

"In particular only those playing principally at Tour level make a decent living. Only the top 250 female players and the top 350 men players break even before coaching costs, yet there are around 15,000 professional players."

The review panel was set up in February 2016 following allegations made by the BBC and Buzzfeed that leading players, including grand slam winners, were involved in suspected match-fixing and that evidence had been suppressed.

Having interviewed around 3300 players as well as tournament organisers, officials and betting operators, it found "evidence of some issues" at grand slams and Tour events, although it did not uncover evidence of a widespread problem at those higher levels.

The panel found no evidence of top-level players being implicated in corruption.

However, it did claim "tanking" – players seemingly giving up during matches – which has been a feature at some high-profile tournaments, has been too often tolerated by the tennis authorities.

The level of suspicious betting alerts rose sharply after the sale of official live scoring data to betting companies in 2012, making tens of thousands of matches available to gamble on.

Lewis added: "The imbalance between prize money and costs, and deliberate under-performance, are the seeds for corruption.

"It is a small step for a player who already intends to lose for other reasons, to bet or to make others aware of their intentions. It's a small step to deliberately lose, or lose a game or a set, so as to make enough money to continue playing.

"According to experts, since 2015 tennis has been responsible for more suspicious betting than any other sport."

Among the panel's recommendations is the restructuring of the professional game with a significant reduction in tournaments deemed 'professional'.

The panel also recommended discontinuing the sale of official live scoring data at lower-level tennis – although betting companies claim this will merely encourage black market activity – and eliminating betting sponsorship in the sport.

The review did not find evidence of a cover-up by either the Tennis Integrity Unit nor the International Tennis Federation and the Association of Tennis Professionals – a finding welcomed by the governing bodies.

However, some of the actions taken by the ITF and ATP were seen to be "inappropriate or ineffective".

Nevertheless, a joint ATP, ITF, Women's Tennis Association and Grand Slam Board statement read: "Following an initial review of the interim report we confirm our agreement in principle with the package of measures and recommendations proposed by the IRP.

"These include the removal of opportunities and incentives for breaches in integrity, the establishment of a restructured, more independent Tennis Integrity Unit, enhanced education, expanded rules, and greater co-operation and collaboration with the betting industry and broader sports community."

Former world No.1 Novak Djokovic, meanwhile, has suffered another early defeat as Slovakia's Martin Klizan won their second round match at the Barcelona Open.

Djokovic has now won just two of five matches since the Australian Open, where he was beaten by Chung Hyeon, after a 6-2, 1-6, 6-3 loss at Sabadell on Wednesday. Klizan converted all three of his break-point opportunities to prevail.

After losing to Dominic Thiem of Austria at the Monte Carlo Masters last week, the 30-year-old Serb experienced another defeat ahead of the French Open at Roland Garros, which begins on May 22.

Djokovic suffered early exits at the Miami Open, losing to Frenchman Benoit Paire earlier this month, and last month at Indian Wells, where Japan's Daniel Taro was the victor. He has not reached the last eight of a tournament since Wimbledon 2017 and has struggled with a long-term elbow injury.

Klizan, a qualifier and world No.140, will meet Feliciano Lopez in the quarter-finals after the Spaniard's win over Serbia's Dusan Lajovic on Tuesday.

Top seed Rafael Nadal and second seed Grigor Dimitrov of Bulgaria experienced no such problems, but Japan's Kei Nishikori retired after losing the first set to Guillermo Garcia-Lopez. Nishikori had an issue with his right wrist.

Nadal beat fellow Spaniard Roberto Carballes Baena 6-4 6-4 to win his 37th and 38th consecutive sets on clay, breaking a record he had jointly held with Ilie Nastase of Romania.

Nadal has now won 15 successive matches on the surface and next plays Garcia-Lopez.

Dimitrov defeated France's Gilles Simon 6-2 6-1, while Thiem, the third seed, defeated wildcard Jaume Munar of Spain 7-6 (8/6) 6-1.

Dimitrov will next play Malek Jaziri of Tunisia.