"Andy has to be realistic, put the facts down on the table: 'This is what I do, this is what it takes to win. Can I be a winner just playing baseline against [Novak] Djokovic?' It's not going to be that easy.
"Then you've also got [Milos] Raonic, [Grigor] Dimitrov, [Bernard] Tomic. You've got eight or 10 guys [coming through], including a couple from Australia. But Andy's physically fit and I believe that his mental attitude this year is excellent."
Sousa, Murray's next opponent, has not won a set in his three matches against the Scot, including their second-round meeting here two years ago. The world No55, who enjoyed a good victory yesterday beating the No32 seed Martin Klizan, is based at the Barcelona academy run by Francisco Roig, who also works occasionally with Rafael Nadal.
"He's had some decent wins in Slams before," Murray said. "He's normally done pretty well. He beat Dimitrov at the US Open [two years ago]. He fights hard. He has a great attitude. He doesn't have one massive weapon, but he definitely gets the most out of his game. He's physically in very good shape. He lasts the long matches well. He'll make it tough for me."
Murray beat Matosevic in just an hour and 42 minutes, having taken immediate control by winning 21 of the first 24 points. The Scot dominated the rallies, served well, returned superbly and made only 12 unforced errors to Matosevic's 49.
"I felt like I struck the ball a bit better today than I did in my first round," Murray said. "I think I moved pretty well. I defended well."