Siddharth has stated that direction was his "ultimate dream". After finishing his education at age 21, Siddharth landed the role of assistant director for Mani Ratnam?s Kannathil Muthamittal. The script writer of that film, Sujatha, was insistent that Siddharth auditioned for S. Shankar?s Boys which would feature five debutant actors. After consulting with Mani Ratnam, he met Shankar, auditioned, and got the role of Munna. The film was a moderate success in Tamil but the dubbed version in Telugu gave Siddharth an opening in the Telugu film industry. Siddharth then appeared in another Tamil film, Mani Ratnam?s Aayutha Ezhuthu (2004), after which he would shift focus and concentrate on Telugu-language projects. His first direct Telugu project, Nuvvostanante Nenoddantana released in early 2005. The comedy-drama film, marking the directorial debut of choreographer-turned-actor Prabhu Deva, became a commercial blockbuster as well as a critical success, with Siddharth winning his first Filmfare Best Actor Award (Telugu) for his portrayal of Santhosh, a playful NRI, who fights for his love. Siddharth, subsequently, also ventured into the writing forum as well as the singing forum as he wrote the screenplay for his next Telugu film Chukkallo Chandrudu (2006) directed by Siva Kumar and sang as a playback singer. He next debuted in Bollywood as part of the critically acclaimed blockbuster Rang De Basanti which also starred Aamir Khan. Siddharth received the Star Screen Debut Award for his role as Karan Singhania in the film. Siddharth?s final release of 2006 was Bommarillu, directed by then newcomer Bhaskar. The film, revolving around a father-son relationship, received critical acclaim and garnered box office success, too. He worked with director Vishal Bhardwaj and Oscar winning cinematographer Guillermo Navarro on a short film titled Blood Brothers, which premiered at the 2007 Toronto International Film Festival as part of Mira Nair?s AIDS Jaago project.
After the above average grossing Aata in 2007, Siddharth took a sabbatical and did not have a release for 20 months. In 2009, his Konchem Ishtam Konchem Kashtam opposite Tamannaah received good responses from critics and audiences. Siddharth played the lead role in debutant director Anand Ranga?s Oy!, which achieved box office success despite mixed reviews. His first release of 2010 was Striker, his second Bollywood venture that portrayed the life of a carrom player. It became the first Indian film to be released in theatres and on YouTube on the same day. His other release that year was the Telugu film Baava, a village-based romantic family entertainer, shot in places like Palakollu, Razole, Rajahmundry and Hyderabad. His first release of 2011, Prakash Kovelmundi?s fantasy epic Anaganaga O Dheerudu was produced by veteran K. Raghavendra Rao. He was next seen in the Tamil-Telugu bilingual 180 by Jayendra, which performed poorly at the box office. His third release of the year was Oh My Friend, directed by debutant Venu Sriram, which opened to mixed reviews but went on to garner positive response. In 2012, Siddharth first starred in the bilingual romantic comedy Kadhalil Sodhappuvadhu Yeppadi (Tamil) / Love Failure (Telugu), based on the same-titled short film by Balaji Mohan, which fetched critical acclaim. He has signed up a Tamil film to be directed by an associate director of Vetrimaaran.