Murray has already won two titles this season
Andy Murray will face a tough test in his opening match at the Sony Ericsson Open after Mario Ancic came through the first round in Miami on Thursday.
Murray, the world number 13, had a bye in the first round and will open against Ancic after the Croatian beat Gilles Simon 6-2 6-7 (2-7) 6-3.
Ancic is currently ranked 63 after missing much of last year through illness but has been as high as seven.
Marat Safin slumped to 1-5 for the year with a loss to Bobby Reynolds.
"I'm trying, and it doesn't come out my way," said Safin, who was beaten 7-6 (7-3) 4-6 7-6 (7-5) by the American qualifier.
"But (that) doesn't mean that I have to stop because I'm not winning matches. I enjoy what I'm doing. I have plenty of cash to do what I want to do, so what I want to do is play tennis."
I feel fine now. I'm motivated and I'm confident on the practice courts
Mardy Fish, who beat Roger Federer on his way to the Indian Wells final last week, sufferered a surprise 6-3 6-3 defeat by France's Arnaud Clement.
Murray and Ancic have met twice before, with the Scot beating Ancic in the Marseille final last month, and Ancic winning in Auckland in 2006.
Miami boasts a strong field as a Masters Series event, with world number one Roger Federer facing France's Gael Monfils in round one.
Federer has yet to win a title in 2008 after struggling with glandular fever, and he lost to Fish in the Indian Wells semi-finals last week.
"You move on, you analyse it, then you look forward," said the Swiss. "I feel fine now. I'm motivated and I'm confident on the practice courts."
Defending champion Novak Djokovic, who plays qualifier Kevin Anderson, said he was eager to build on his good run of form having already achieved two of his goals set for this season.
The Serb won the Australian Open in January and took the Indian Wells title last week.
"It has been a dream start, of course," said Djokovic after his 6-2 5-7 6-3 victory over Fish in the Masters Series final on Sunday.
"Before I started playing this season, I said that my two goals are to be consistent with the results, reach the Masters Cup and hopefully win one Grand Slam.
"I did all that in a very, very small amount of time."
The 20-year-old Serbian added: "I don't want to stop here. I'll try to keep going in Miami, and I'm defending champion there. Coming up from Indian Wells as the tournament winner, it's much easier."
The women's event sees the return to action of world number one Justine Henin, along with Serena and Venus Williams and Ana Ivanovic.
In-form Maria Sharapova pulled out of the event with a shoulder injury.