Murray is ranked 113 places above Alex Bogomolov of the United States
Andy Murray lost to a qualifier in a second consecutive tournament after a straight-sets defeat in the Sony Ericsson Open by Alex Bogomolov Jr.
Murray was broken seven times as the world number 118 secured a 6-1 7-5 victory in an hour and 35 minutes.
The loss comes two weeks after Murray was dumped out the BNP Paribas Open at Indian Wells by Donald Young.
And the Scot has failed to win a single set since defeating David Ferrer at the Australian Open in January.
"I've played poorly last two weeks, but I was happier with the way that I competed this week than last. Confidence could have something to do with that," he said after his latest loss.
"In practice I have been competing well," he added on his official website.
"Especially this week, chasing everything down, playing a lot of good points and feeling good, then in matches I haven't been able to get it going at all.
"It's happened to me in the past where I've had bad moments and I've come back well from them."
Murray, the number five seed, unveiled Dani Vallverdu as his new full-time coach on Friday, but this failed to provide an immediate boost to his form.
He served three aces against none by Bogomolov, but won just 44% of his service points - and only 33% on his second serve - compared to his American opponent's 58%.
Murray was broken in the very first game of the match, and again in game three, before holding game five to make it 4-1.
But Bogomolov broke Murray's serve for a third time to take the first set 6-1.
The British number one started the second set more positively, and broke his opponent's serve for the first time in the opening game of the set.
But the following three games also went against serve, before both players held their next service games to make it 3-3.
Bogomolov broke again to lead 5-3, before Murray broke back then levelled the set at 5-5.
But he could not hold on to his next service game, allowing Bogomolov to claim victory with his seventh break of the match.
The latest loss, following the straight-sets defeat by Young in the second round at Indian Wells, raises fears that Murray is suffering a similar slump in form to that of this time last year.
After losing to Roger Federer in the 2010 Australian Open final, he did not make another final until the Farmers Classic in Los Angeles in August, where he lost to Sam Querrey.
The other notable upset of the day in Miami came as Pablo Andujar saw off his fellow Spaniard and ninth seed Fernando Verdasco 3-6 7-6 (7-3) 6-4.
Swiss 12th seed Stanislas Wawrinka was ousted from the tournament by another Spaniard, Marcel Granollers, losing 6-0 6-7 (4-7) 6-3.
Another seed, 24th-ranked Spaniard Guillermo Garcia-Lopez lost 6-3 5-7 6-4 to South Africa's Kevin Anderson.
India's Somdev Devvarman, who had a good run to the fourth round in Indian Wells, beat 31st seed and rising Canadian star Milos Raonic 7-6 (7-5) 7-5.
And James Blake gave the home fans more to cheer after Bogolomov's win by beating Brazilian 27th seed Thomaz Bellucci 2-6 6-4 7-6 (8-6).