That is the verdict of Tennis Channel pundit Jon Wertheim.
The BNP Paribas Open has throw up some surprise results so far and Federer remains the only top four seed player left in the tournament.
Federer takes on Jeremy Chardy later today and needs to reach the semi-finals in order to retain his World No 1 ranking.
Del Potro meanwhile, has reinvented his game and is back in the top 10 in the world.
The number six seed secured his place in the last 16 after a straight-sets 6-4, 7-6 win over Spanish veteran David Ferrer.
And Wertheim thinks that Del Potro will be the only player who won’t have any concerns over Federer.
“There’s one player in this draw who doesn’t fear Roger [Federer],” Wertheim told the Tennis Channel.
“Circle the name Del Potro.
“Keep an eye out for him.”
Del Potro has made an impressive start to the season and he won the Mexican Open earlier this month.