Murray was broken only once by the Paraguayan

The Scot, seeded eighth at the Legg Mason Classic, began working with Gilbert this week at the ATP event.

Murray, 19, took the first set in just under 50 minutes, with a break in the third game proving decisive.

After the pair swapped breaks early in the second set, Murray broke again in the fifth but needed two chances to serve out the victory.

After coming through in fierce heat in one hour and 33 minutes, Murray revealed that his new coach had not been overly impressed.

"He gave me a C for my performance," said Murray. "He said I played a bad service game at 5-2 (in the second set).

I want to win a Grand Slam and you can't do that in the shape I'm in now Andy Murray

"I was maybe a little too defensive with the weather. Working with a new coach, it's always a little difficult.

"You always want to impress him. I was just glad to get through. I think he was grading me on the American scale."

Gilbert said: "He had his guy down and tired and let him back in a couple games. The best thing in life is to win with a C.

"That keeps you around in the tournament."

The American did his best to play down the expectations surrounding his partnership with Britain's best hope.

"Everyone thinks I'm a magician - I can just touch him and everything is going to be great," said Gilbert.

"It's hard work. It takes time. It's methodical. It's just getting on the treadmill and going forward.

"He has to get a lot stronger and fitter. You can't get it in five minutes. It's going to take time. It's a journey for the rest of his career.

"He has to get better in a lot of areas. The two I see most are his serve and fitness. If he can get in the top 10 in service games held, you are going to see great things from Andy Murray."

Murray added: "I realise I have to do that. I want to put the hard work in. I want to win a Grand Slam and you can't do that in the shape I'm in now."