Proper fitting wrap around sunglasses keep light from entering from the top, sides and and bottom.
Don't have a nose piece that is too big or thick frames that can block any of your view.
Brown maximizes contrast in bright light. Yellow maximizes contrast in low light. A yellow- brown or amber lens seems a good all around color.
Very dark grey lenses can make a tennis ball appear as a dark blob on the side of the court opposite the sun.
Polarized lenses block the glare off of hard courts, fences and vehicles parked on the other side of the court (if no wind screen is present.)
Many think Oakley's have the least distortion, and would be a good choice if you've got the dough. (I like the Half-Jacket XL pictured above, but it is not for everybody.)
For prescription wearers who don't use contacts, single vision rather than progressive/bifocal lenses seem to work best - the progressive/bifocal distorts rather than helps on balls in the lower part of the lens, and your contact point is too far away to need near vision correction.
No question that bright sunlight increases your risk for cataracts later in life - most sunglasses block virtually all of the harmful ultraviolet rays.
A hat with a brim keeps the sun off of the lenses, decreasing glare even further.
A dark underside of the brim keeps reflected light from bouncing off the brim and down to your sunglasses/eyes.
If you play doubles, another advantage to a righty-lefty combination - neither will ever have to serve into the sun (unless it's a really long match starting around noon).
Keep working on your slice and kick serves. Sometimes the sun is just too tough to hit your usual first or second serve - but you can still win with a ball toss to the left or right.
Definitely practice serving in the sun to gain experience.