SAO PAULO -- There was finally an opponent Ronaldo couldn't overcome -- his own body.
No longer able to meet the demands of football because he can't stay fit anymore, Ronaldo announced Monday that he is retiring, ending a stellar 18-year career in which he won two World Cup titles with Brazil and thrived with some of Europe's top clubs.
"It's very hard to leave something that made me so happy," a tearful Ronaldo said. "Mentally I wanted to continue, but I have to acknowledge that I lost (the fight) to my body."
The 34-year-old striker, who has scored a record 15 World Cup goals, said a string of injuries in the past two years had kept him from performing at a high level with Brazilian club Corinthians.
"The pain made me anticipate the end of my career," Ronaldo said, with sons Alex and Ronald by his side. "It hurts when I go up the stairs, people who are close to me know this. I've given my life to football. I don't regret anything, but I can't keep going."
Ronaldo blamed the excessive number of matches and practice sessions for his physical decline, saying it's inevitable players will be similarly affected. The striker said he injured a muscle last week, which was the final straw.
"I thought about it at home and realized that it was time," he said. "I had given everything that I had."
Ronaldo also said that he found out four years ago that he had hypothyroidism, a condition that made it difficult for him to lose weight and stay in shape.
With his uncanny sprints toward goal and an incredibly accurate finishing touch, he captivated fans everywhere he played, including Barcelona, Real Madrid and Inter Milan.
All despite having to come back from three serious knee injuries that threatened to end his career earlier than expected.
"My career was beautiful, was wonderful," he said. "I've had many defeats but infinite victories."
Ronaldo's contract with Corinthians was expected to expire at the end of the year, but his physical condition kept deteriorating. Visibly out of shape, he was not able to perform well and fans were jeering him after every missed play.
The announcement came just days after he was heavily criticized by Corinthians fans for the team's elimination from the Copa Libertadores, Latin America's most important competition and the only major tournament the Brazilian club has yet to win.
Supporters damaged players' cars and threw rocks at the team's bus, but Ronaldo said the violent protests did not play a part in his retirement. He said he only wanted to thank Corinthians fans for their support.
"I've never seen fans with so much passion," he said, his voice cracking. "Their need for results sometimes made them a little aggressive, a bit out of control."
Ronaldo's exit comes two days after former Brazil teammate Roberto Carlos left Corinthians because he and his family were being threatened by fans after the Copa Libertadores elimination. Last year, Corinthians failed to win any titles in its centenary year.
"I want to publicly apologize for failing in the Libertadores project," Ronaldo said, crying, before team president Andres Sanchez handed him a jersey with the words "forever" and "phenomenon."
The last of Ronaldo's more than 400 career goals came from a penalty in a 1-0 win over Cruzeiro in the Brazilian league on Nov. 13. The last match was the 2-0 Copa Libertadores loss to Deportes Tolima on Feb. 2 in Colombia.
"I'll miss it a lot," Ronaldo said, without talking much about his plans for the future. "With this announcement, it feels like it's my first death."
Ronaldo was a member of Brazil's World Cup squad that won the 1994 World Cup in the United States, although he was a teenager and never played. He was the team's top star in France in 1998, but just before the final he suffered seizures at the team's hotel and disappointed as the French won.
"Ronaldo is the best player I ever played with," former Inter Milan teammate Youri Djorkaeff, who also played against Ronaldo in the 1998 World Cup final, told French RMC radio. "But he was also a very engaging person. When we were training, he would always come up with crazy dribbles. We would practically stop to watch him. It was extraordinary."
It was in 2002 that Ronaldo peaked with the national team, helping Brazil win the tournament in South Korea and Japan with two goals in the final against Germany.
He clinched his third FIFA Player of the Year award that year, adding to the ones he had already won in 1996 and 1997.
He was among those who disappointed in the 2006 World Cup in Germany, when Brazil fell to the French again in the quarterfinals, but he was still able to score his record-breaking 15th goal, a mark that still stands.
Ronaldo also helped Brazil win the 1997 and 1999 Copa America, as well as the 1997 Confederations Cup and a bronze medal at the 1996 Atlanta Olympics.
Ronaldo scored 67 goals in 104 internationals with Brazil, but was never again called up to the national team after the disappointing performance in 2006. There were calls for his return to the squad after he helped Corinthians win the 2009 Brazilian Cup, but he was never able to get back in shape.
Ronaldo's career was marked by his goals just as it was by his injuries, which kept him sidelined for nearly three years in total.
He tore up his right knee with Inter Milan in 1999 and needed surgery, and a year later he twisted the same knee on the day he was returning to action and had to be sidelined for several months again after another surgery. The third injury came with AC Milan in 2008, forcing another surgery and another long layoff.
"He played past injuries what would have made many other players quit," said 30-year-old Hamilton Pereira Felix, a Rio de Janeiro doorman glued to the television waiting to hear Ronaldo's announcement. "He was a warrior. But he couldn't recover anymore. It was his time."
Ronaldo began his professional career with Cruzeiro in 1993 as a 16-year-old, and it didn't take long for him to start stunning fans across Brazil. He moved to PSV Eindhoven that same year, becoming the club's top scorer and earning a transfer to Barcelona in 1996.
He quickly became an idol at the Spanish club, scoring 34 goals in 37 matches in the Spanish league and helping the club win the Copa del Rey.
A year later he signed with Inter Milan, winning the 1998 UEFA Cup and earning the nickname "The Phenomenon."
He moved to Real Madrid in 2002, and is one of the few players to become an idol for fans of both Spanish powerhouses. He helped Real Madrid win the Spanish league in 2003 and 2007. After that season he joined AC Milan, but the third injury in his knee cut short his stay at the Italian club.
Ronaldo is already a goodwill ambassador for the United Nation's Development Program, and he was key for the organization of the "Game for Peace" in Haiti in 2004, when the Brazilians visited the country devastated by gang wars.