At the beginning of 2012, Errani decided to change her racquet, switching from Wilson to Babolat, opting for a heavier and slightly longer model than the last, allowing for more power and better reach. This change caused her to return her $30,000 endorsement fee to Wilson. However, she and several commentators cite the new racquet as a reason for her improved game and her entry into the top ten of the WTA rankings. In the first five months of 2012, she won three singles titles, earning over $1.3 million in prize money. Errani herself dubbed her new racquet "Excalibur", named after the sword of King Arthur.
At the 2012 Australian Open Errani advanced to her first grand slam quarterfinal, defeating Nadia Petrova, Sorana Cîrstea, and Jie Zheng en route before losing to Petra Kvitová. Her ranking jumped to world no. 33, a career best.
At the Abierto Monterrey Open, she was the second seed where she reached the semi-finals, losing to eventual champion Tímea Babos, but she triumphed on the clay in Acapulco as the third seed, winning her third career title. She defeated fellow Italians Roberta Vinci, and second seed Flavia Pennetta in her last two matches.
Errani fell in the opening round of Indian Wells to Vania King, and in the second round of the 2012 Sony Ericsson Open against Sloane Stephens.
As the 7th seed at the Barcelona Ladies Open, Errani stormed to her fourth career title and second of the year, not dropping a set, beating second seeded Julia Görges in the quarterfinals, Carla Suárez Navarro in the semifinals, and Dominika Cibulková in the final. Afterwards, her ranking rose to world no. 28. In the Fed Cup semifinals against the Czech Republic, Errani lost to Petra Kvitová, but beat Andrea Hlaváčková, with Italy losing the tie.
On May 7, 2012 Errani won the 2012 Budapest Grand Prix, tying countrywoman Roberta Vinci for the most titles won by an Italian female in a year at three.
Her performance continued to improve when she reached the finals of the 2012 French Open. En route she defeated Casey Dellacqua, Melanie Oudin, and 2008 and 2009 French Open winners Ana Ivanovic and Svetlana Kuznetsova, respectively. She secured a place in the semifinals by eliminating the German tenth seed Angelique Kerber, her first ever win over a current top-10 player. In the semifinals, she overcame the reigning US Open champion and 2010 French Open finalist Samantha Stosur to reach her first major singles final opposite Maria Sharapova. Errani lost in the final. However, her progress in this tournament helped her achieve the no. 10 ranking.
In addition to her singles wins, Errani also won six doubles titles with Roberta Vinci including the 2012 Mutua Madrid Open, the 2012 Internazionali BNL d'Italia and the 2012 French Open. She and Vinci also made the finals of the 2012 Sony Ericsson Open and the 2012 Australian Open.
At Wimbledon, Errani lost in the third round to Yaroslava Shvedova, suffering the humiliating loss of the first Golden Set (i.e. a set in which every point is won by the same player) in the women's tennis open era, and second ever recorded. In the month of July, at the Italiacom Open 2012, Errani, without losing a set, won her fourth title of the year, the sixth in her career. She defeated the Czech Barbora Záhlavová-Strýcová (seeded eighth) in the final.
Errani then lost in the first round of the singles competition at the 2012 Summer Olympic Games in London.
She then played the 2012 Rogers Cup where she lost in the third round to eventual finalist Li Na. She then lost to Venus Williams in the third round of the Western & Southern Open. She then played the New Haven open as the fourth seed. She reached the semifinals by beating fifth seed Marion Bartoli. She lost in the semifinals to Petra Kvitová.
At the US Open, Errani had a slow start by beating Garbiñe Muguruza Blanco in three sets. However, she easily defeated the Russians Vera Dushevina and Olga Puchkova. She only lost three games in these two rounds. In the fourth round, she defeated her first seed. She defeated the sixth seed, Angelique Kerber. In the quarterfinals, she defeated her doubles partner, Roberta Vinci, to come into the semifinals, where she lost in straight sets to eventual champion Serena Williams. With this result, she is the first Italian woman in the Open Era to reach the semifinals of the US Open, and the first Italian woman ever to come at least into semifinals of two different majors.
Errani partnered again with Vinci for the US Open. They were seeded second, behind the defending champions Liezel Huber and Lisa Raymond. Errani and Vinci won the US Open women's doubles final. As of result, she reached the no. 1 spot on 10 September 2012.
At the end of the year she took part in the WTA Tour Championships for the first time in her career. Here, she was defeated in straight sets by Maria Sharapova, but she beat Samantha Stosur in the following match; she lost to fourth seed Agnieszka Radwańska in a match lasting three hours and 29 minutes (the longest best-of-three-set match in WTA Championships history). With that loss, Errani did not advance beyond the pool stage. She finished her breakthrough year as no. 6 in the world in WTA singles rankings.