In general serving in doubles is way different then in singles. Winning free points is not always guarenteed and you could be taking your partner out of the point (either by winning on the serve or missing serves) which makes you a weaker team and makes the game where the singles player is serving the weaker game.
And depending on the conditions or the mentality of the players sometimes switching serving ends can be a issue. You have to get used to an entirely different side which in the 3rd set could be daunting.
That being said though, these are pros and the game is way different for them. Some singles players are WAY better then some of the regular doubles players in terms of skill so you can throw general theory out the window.
Commentators dont always pick up on that and some of those older players that did both dont seem to understand that the game has changed from the days where many players did both.
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