Venus Williams will make her comeback in an exhibition match against her sister, Serena, in Colombia on Wednesday, two months after pulling out of the US Open because of an immune system disease. Venus told Thursday's edition of the Italian newspaper, Gazzetta dello Sport, that she has changed her diet to increase vegetables after being diagnosed with Sjogren's syndrome, which can cause fatigue and joint pain. After playing Serena in Medellin next week, Venus will join her sister against the Italians Francesca Schiavone and Flavia Pennetta in another exhibition match on December 3 in Milan. Serena also has not played since losing to Samantha Stosur in the US Open final in September. "I did a lot of medical therapy and a serious evaluation of my diet to understand which foods help control the symptoms better," Venus Williams told the Gazzetta. "And that's why I've completely changed my diet and filled it with a lot of vegetables. In the meantime, I've continued to keep myself fit with tennis and in the gym. I'm very confident in my progress." Venus's world ranking has fallen to No 103 because of injuries. She is planning to return to the WTA Tour in January at the ASB Classic in Auckland, New Zealand. "The London Olympics are a big goal," the five-time Wimbledon champion said. "I would like to continue playing at my best level. "I love tennis and the racket seems like it was made explicitly for my hand. "Within the next 12 months, I'm expecting to get back to the top of the rankings in singles and doubles. I want to play all year long. I've still got a lot of work to do but it doesn't scare me - I'm used to it."