Assuming they serve roughly as often as they return (since it alternates) we have an average win% of 59.5%, which tallies up with the OP.
As many have commented above the nature of what constitutes a win in tennis vs SC2 makes up for a lot of the differences, and as in almost all sufficiently deep competitions the differences at the top are so small that to eek out even a small advantage will put someone 'way out in front' even if their win ratios are never in the 80 or 90% range.
