The duo, who have won 36 Grand Slams between them, have dominated the rankings for years.
Federer, 36, is the man who currently holds the top spot but he was beaten in Indian Wells by Juan Martin del Potro on Sunday.
Shiras and Croft have now backed the Argentine to take aim at the big guns after fully recovering from a wrist injury.
Croft said: “I think we all saw the quality when he was a young teenager and he burst on to the tour.
“Everybody knew this explosive, exciting games he possessed.
“Even in the final, it was quite amazing because we all love to watch Federer’s forehand, but when Del Potro was connecting with his, especially on the running forehand down the line, the oohs and the aahs from this crowd… They were absolutely gobsmacked by the sheer pace.
“It’s the timing he has when he connects with the ball, it was a sight to behold.
“It’s an unusual shot, a big backswing, but it’s the connection with it.
“Federer was slicing the ball, they were bouncing off the ground by an inch but Del Potro was still able to get his racket face under the ball to hit winners. It’s quite extraordinary.
“I’m not sure he’s ready to take Roger’s tag on the top of the game but he’s definitely putting himself in the mix.”
Shiras agreed and said: “He’s got a huge strikezone. He can hit it when it’s low, he can play it when it’s high. He’s a great volleyer.
“I’m surprised he doesn’t come to the net more with the way he reads balls and makes instinctive plays at the net.
“I feel like this guy is on a trajectory upwards.
“He’s going to be in the top five and maybe make a run even higher.”