Of course we know how important holding your serve is at the pro level, where a break often means the set. But one of the other active threads about holding when serving first got me thinking... how much of a big deal is holding serve at the club level (3.0 - 4.5)? My personal experience, mainly from playing men's 4.0 singles, is that it's pretty common to trade 2 breaks per set. Meaning that I've often reached scores of 6 all (heading into a tiebreak) where I won 4 service games + two breaks, as did my opponent. Also, winning a set 6-4 might also often include a couple of breaks each way. Based on this, I would say that at 4.0 singles, the server wins the game maybe 2/3 of the time on average. Definitely an advantage, but not overwhelming. So I wonder - what are the %'s of service holds at other levels and in doubles versus singles?