Sania Mirza and her American partner Bethanie Mattek-Sands suffered a straight-set defeat against the pair of Kirsten Flipkens and Laura Siegemund in the first round of the Abu Dhabi Open here.The Sania-Bethanie duo lost 3-6, 4-6 to their Belgian-German opponents in the match that lasted one hour and 13 minutes Monday night to make an early exit from the tournament.Sania, a six-time Grand Slam winner, is set to retire from professional tennis after playing in the Dubai Tennis Championships beginning February 19.Last month, Sania and Rohan Bopanna had finished runners-up in the mixed doubles in the Australian Open in Melbourne as they lost to Brazilians Luisa Stefani and Rafael Matos in the final.