Martina Navratilova, John McEnroe and Rod Laver are three members of tennis royalty, who also just happen to be left-handed [source: Smith-Johnson]. For many years left-handed players held a distinct competitive advantage, and probably still do in lower level competition. The reason was tennis pros were told to serve from the left which aimed toward the opponents' backhand, typically their weaker side. In the modern era, however, it's rare to encounter a weak pro backhand, so some of the lefty edge has been mitigated [source: Liew]. In fact, the ATP 2017 top 35 men's rankings only feature a handful of left-handed players, including Rafael Nadal and Feliciano Lopez. (Interestingly, Nadal was actually born right-handed, but at some point in his childhood training, decided to use his non-dominant left hand to primarily handle his racquet.)
Lefty female players like Angelique Kerber, Lucie Šafářová and Petra Kvitová have all enjoyed significant success in recent years; however they remain outnumbered by righties in the rankings [source: Liew].
For the moment, however, recreational league level lefty tennis players continue to have a bit of an edge over righties, if for no other reason than they have more opportunities to face down right-handed opponents. In fact, a small study of equally divided right- and left-handed tennis players at a range of levels found that all groups were better able to anticipate the directional strokes when facing a right-handed player [source: Hagemann].
The left-handed advantage holds true for other one-on-one sports, like boxing and fencing, and is known as the fighting hypothesis that says that left-handedness has persisted evolutionarily because it gave its owners a fighting advantage [source: Jarrett].