UFC 61: Bitter Rivals was a mixed martial arts event held by the Ultimate Fighting Championship on Saturday, July 8, 2006. The event took place at the Mandalay Bay Events Center, in Las Vegas, Nevada and was broadcast live on pay-per-view in the United States.

The main event was a Heavyweight Championship match between champion Tim Sylvia and former champion Andrei Arlovski. This was the third time these fighters had met, they had split the first two meetings. The co-main event was the highly anticipated rematch of TUF 3 coaches Tito Ortiz and Ken Shamrock.

It was announced during the event that Chuck Liddell would fight PRIDE star Wanderlei Silva in November if Liddell won his upcoming match against Renato "Babalu" Sobral at UFC 62. Negotiations between the UFC and PRIDE failed however and the Liddell-Silva fight never occurred in November 2006. The two would eventually meet in December 2007 at UFC 79.

The number of pay-per-view buys for UFC 61 in the United States was approximately 775,000, which set a new UFC buyrate record and generated gross PPV revenue of approximately $30,960,000. The disclosed fighter payroll for the event was $676,000. With a live gate at $3,350,775, it nearly eclipsed the all-time mark for UFC events set at UFC 57.

Kenneth Wayne Shamrock (born Kenneth Wayne Kilpatrick; February 11, 1964) is an American mixed martial artist, UFC Hall of Famer and professional wrestler. Shamrock emerged as one of the biggest stars in the history of mixed martial arts, headlining over 15 main events and co-main events in the Ultimate Fighting Championship and Pride Fighting Championships during the course of his career and set numerous pay per view records with his drawing power. Shamrock is widely considered to be a legendary figure and icon in the sport of mixed martial arts. Shamrock was named The World's Most Dangerous Man by ABC News in a special entitled "The World's Most Dangerous Things" in the early part of his UFC career, a moniker which has become synonymous as his nickname.

Shamrock became known early on in the UFC for his rivalry with Royce Gracie. After fighting to a draw with Gracie in the inaugural Superfight, he became the first UFC Superfight Champion after defeating Dan Severn at UFC 6; the title was eventually renamed the UFC Heavyweight Championship when weight categories were introduced to the UFC. He was also the first foreign MMA Champion in Japan, winning the title of King of Pancrase. During his reign as the UFC Superfight Champion, he was widely considered the #1 mixed martial artist in the world. Shamrock was also ranked by Inside MMA as one of the top 10 greatest mixed martial arts fighters of all time.

Jacob Christopher "Tito" Ortiz (English: /tɪtɵ ɔərtɪs/) (born January 23, 1975) is an American mixed martial artist and former UFC Light Heavyweight Champion, having held the title from April 14, 2000 to September 26, 2003. Along with fighters like Randy Couture and Chuck Liddell, he was one of the sport's early stars. Ortiz ultimately became the biggest pay-per-view draw of 2006 for his fights with Liddell, Forrest Griffin, and Ken Shamrock.

Outside of his fighting career, Ortiz is the CEO of the Punishment Athletics MMA equipment and clothing line. He also owns an MMA training gym called Punishment Training Center, which is located in Huntington Beach, CA.

Jacob Christopher Ortiz was born to Samuel and Joyce Ortiz. He received his nickname "Tito," which means tyrant, from his father when he was a year old. He is the fourth child of his mother, Joyce, who had three sons from a previous marriage. His mixed heritage (Mexican father, American mother) has been reflected in his ring entrances as he has borne both Mexican and American flags.

Timothy Deane Sylvia (born March 5, 1976) is an American professional mixed martial arts (MMA) fighter, professional wrestler, and former two-time UFC Heavyweight Champion. He trains as a member of the Miletich Fighting Systems camp and is part of the Miletich Fighting System Elite group which has had members such as former UFC champions Matt Hughes and, Jens Pulver. Sylvia was promoted to a purple belt in Brazilian Jiu-Jitsu on October 31, 2011. He holds notable wins over Andrei Arlovski (x2), Jeff Monson, Ricco Rodriguez, Paul Buentello, Brandon Vera, Jason Lambert and Ben Rothwell

Sylvia joined a karate school as a child, and began wrestling in high school. After graduation, he worked construction, community gardening, hanging sheet rock, a bouncer for Shane Reids bar, and painting houses. He was playing semi-pro football when he became interested in MMA, and began to train with Marcus Davis. During this time he decided to take up boxing and grappling. After training for a year, he got the opportunity to fight in a Rhode Island no-holds-barred amateur event, in which he knocked out his opponent in 17 seconds.

Andrei Valeryevich Arlovski (Belarusian: Андрэй Валер'евіч Арлоўскі, Belarusian pronunciation: [anˈdrɛj vaˈlʲɛrɛvitʃ arˈlɔwski], Russian: Андре́й Вале́рьевич Орло́вский, born February 4, 1979) is a Belarusian kickboxer, mixed martial artist and a former Ultimate Fighting Championship heavyweight champion. He holds notable wins over Fabricio Werdum, Roy Nelson, Ben Rothwell, Tim Sylvia, and Vladimir Matyushenko.

Arlovski was born in Bobruisk and raised in Minsk, in the Byelorussian Soviet Socialist Republic of the former USSR (now Belarus). When he was younger, bullies often picked on him and beat him up. In 1994, when he was 14 years old, he finally had enough and started lifting weights to put on muscle and, he hoped, to help him deal with these bullies. Andrei only took up martial arts at the age of 16 having previously been interested in football. It was an interest in personal fitness and conditioning that led Arlovski to take an interest in combat sports.

Enrolling at the police academy in Minsk, Arlovski combined his interest in a career in law enforcement with his growing martial arts participation by taking up the required police defense course in Sambo and Brazilian Jiu-Jitsu and quickly showed himself to be a highly competent Sambo exponent. In 1999, Andrei won the European Youth Sambo Championship and the World Youth Championship. Not long after, Andrei became the first Master of Sports and then International Master of Sports. He took a silver medal at the Sambo World Cup and another silver medal at the World Sambo Championship.