it certainly wasn't the return that was the problem in that match it was the fact Isner was playing brilliant from the baseline in sets three and four and outhitting Murray, but cause he's an 'all serve clown' you won't admit that.
BTW Djoker needed 5 sets to beat him on an indoor clay court in Davis Cup.
Murray's eaten Karlovic's serve up on a few occasions, whereas Djokovic hasn't. He hasn't faced any big servers this year and his return stats are skewed by facing clown serves like Nadal's.
Djokovic broke Tsonga at will sometimes at Wimbledon 2 days after your GOAT had only 1 break point