Cape Town - Sri Lanka's vice-captain Angelo Mathews believes spinner Ajantha Mendis will be one of their trump cards in the upcoming Test series against South Africa.Gallery: SA prepare for Sri LankaMendis was a surprise inclusion in their touring party after a back injury kept the 26-year-old from the Test scene since May this year.He struggled for the tourists in their three-day match against an SA Invitational XI in Benoni over the weekend, taking only two wickets while conceding 142 runs in 31 overs.However, Mathews is not worried about his charge's slow start to the tour and is confident he could shine in the first Test, starting at SuperSport Park in Centurion on Thursday."Ajantha was injured for longer than two months and has only recently started bowling again."Everyone knows what he's capable of and that he could change the face of a game in the blink of an eye." Mendis is notorious for bowling the "Carrom" delivery, where he grips the ball with his thumb and middle finger.And Mathews feels that, together with Mendis and left-arm spinner Rangana Herath, they will have enough to trouble the home side in the series."I'm sure Mendis will be valuable in the series, because spin bowling always plays an important role and in him and Herath we have two world class spinners."We just have to be patient and give Mendis enough time to regain his best form," said Mathews.