"To be honest, it hasn't really happened to me that often where even one guy has done that," said Murray, who had also reached the last 16 without hitting a shot when Canadian Milos Raonic pulled out of their third-round clash with a right ankle injury.
"To get two in one week is strange," Murray said.
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Murray will face either world number one Novak Djokovic or 21st-seeded Juan Monaco of Argentina in the final.
Murray, the 2009 Miami champion, had rallied to beat Serbia's Janko Tipsarevic 4-6, 6-3, 6-4 in the quarter-finals on Wednesday.
Chasing a second title of 2012, to go with the trophy he captured in Brisbane, Murray said he wasn't sure if the extra rest afforded him by Nadal's sore left knee would be good or bad for him come the final.
"You don't really know the answer to that," Murray said. "You never know how you're going to play on the day.
"Obviously, you don't play a match for three or four days, it's different than if you played one one or two days ago.
"But then the positives are that you're going to be fresh and physically feeling fine.
"I don't really know how I'm going to feel for the final, but I'll definitely be fresh."
Djokovic and Monaco, meanwhile, will both be playing for the second day in a row, after booking their semi-final clash on Thursday.
Djokovic, the defending champion, defeated fifth-seeded David Ferrer 6-2, 7-6 (7/1) and admitted he was pleased to finish off the tenacious Spaniard in two sets after twice going up a break in the second frame only to have Ferrer push him to the tie-breaker.
Monaco, who celebrated his 28th birthday by ousting eighth-seeded American Mardy Fish 6-1, 6-3, will try to follow that up by reaching his first Masters level final.
He has one title to his credit already this season, at Vina del Mar. However, Djokovic has won all four of their career meetings.
The Serbian, vying to reach his first final since defending the Australian Open title in January, sounded an ominously confident note after his quarter-final victory.
Despite his semi-final exits in Dubai and Indian Wells, Djokovic said his game was improving with every match.
"I feel that I am playing better and better as the tournament goes on," he said.
