The 'Russian Roulette' hitmaker reportedly flew back to the US from Berlin, Germany, at the weekend just to attend Matt Kemp's Ante Up for Autism charity fundraiser, where she mingled with his friends and relatives.

A source told E! online: "Matt was happy she came. It was an awesome show of support.

"Rihanna fit right in as if they all knew each other. It was comfortable."

The L.A. Dodgers baseball star - whose brother is autistic - recently surprised Rihanna by throwing her a 22nd birthday party.

A source said: "It was so sweet and she was really surprised. She looked just beautiful."

Matt organised Caribbean food to be served to Barbados-born Rihanna and also reportedly bought her a custom-made diamond necklace.

The party was extra-special for Rihanna as she cancelled her 21st birthday celebrations last year in the aftermath of her assault at the hands of then-boyfriend Chris Brown early in February 2009.

The source added: "Matt really wanted to do something nice for Rihanna this year since she didn't have much of a 21st birthday last year. It was just her closest friends, good Jamaican food, a photo booth, dancing and a nice cake."

Matt, 25, has been linked to Rihanna since they were seen holidaying together in Mexico in January, but he has played down romance rumours.

He said: "Rihanna is just a friend, a good friend of mine. We went down to Mexico and had fun together."