Selena Gomez played a show on Sunday night at the Pacific Amphitheatre in Costa Mesa, California. And mid-concert, her fans got a big surprise as Gomez's beau, Justin Bieber, took the stage.

Video posted on Celebuzz.com shows Bieber giving his lady love a big hug before launching into an acoustic version of Justin Timberlake's classic breakup ode, "Cry Me a River." He also reportedly sang "Favorite Girl."

Bieber even took to Twitter to tell Gomez that he thought she did a great job onstage. "Had a #greatnight and surprised the people. felt good to be on stage. nothing is changing that. #REAL," he wrote. "@selenagomez u did great tonight. proud of u."

Bieber, who recently revealed that he was taking a bit of a summer break, also added that as nice as it was to be onstage again, he is even more enthusiastic about the new album he's working on. "More excited than ever to get back in the studio and make this #newalbum," he wrote. "Miss the tour and want to give u all new music. #BELIEVE I do this because i love music. #REAL."

Earlier in the weekend, Go mez celebrated her 19th birthday and offered a big "thank you" to her fans on Twitter. "HappyBirthdaySelena is a trending topic," she wrote. "Thank you all so much for a great birthday. Thank you to everyone who sent birthday wishes, I love you all."