I think I know why Federer and Sampras do better in 5 setters.

Golden Retriever

Hall of Fame
They are two of the greatest servers of all time so they will have no problems holding their service games at all whether for a 3 or 5 setters. However they always have problems breaking a good server because their return especially the 1HBH return is relatively weak. They could only block back serves with a 1HBH and hope the ball goes in deep. Most of the time it doesn't work well and they can't break so Federer and Sampras are never too dominating at their return games. It is not rare to see a nobody taking Federer to 2 tiebreakers. For 5 setters, the block return is bound to work for at least a few games actually even one or two will be enough, in fact it worked only for one game at the Wimbledon final.

On the other hand, for 3 setters, the block return might not be given the opportunity to work if their opponents' serve is on fire that day.

In short, the block return works much better in a 5 setter than a 3 setter.
 
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