Tomic pulls out at ATP Miami with back injury

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Bernard Tomic of Australia has tumbled down 44th in world rankings after being ranked 17th in the world as recently as November

Bernard Tomic's woes continued Thursday as he pulled out of the Miami Open due to a back injury, the latest setback in a disappointing year for the Australian.

Ranked 17th in the world as recently as November, Tomic has tumbled down the rankings to 44th. Since a third-round Australian Open exit, the 24-year-old has lost four first-round matches to lowly ranked opponents.

The most recent setback came at Indian Wells, where he was beaten by 112th-ranked Bjorn Fratangelo of the United States.

At Acapulco in February, Tomic retired from his opening match citing "unbearable heat" and the fact that he withdrew from four ATP matches last year has led to some questioning of his attitude.

Tomic was due to face Kazakh qualifier Mikhail Kukushkin at Crandon Park on Thursday but pulled out shortly before the contest was due to begin.

Lucky loser Mikhail Youzhny of Russia took Tomic's place in the first-round contest, but was ousted 6-4, 6-1.

US teen Frances Tiafoe downed Russia's Konstantin Kravchuk 7-5, 5-7, 6-1 to book a second-round match against 18-time Grand Slam champion Roger Federer. It will be the 19-year-old American's first match against a top-10 foe.

"The guy has about a million Grand Slams and done everything. I can't tell you how excited I am to play," Tiafoe said. "Seeing the draw was partly probably why up a set and 4-1, I started rushing and thinking, 'Oh my god, I'm going to play Roger.'

"He's my idol, everyone's idol."

World number 41 Marcel Granollers of Spain fell 7-6 (8/6), 4-6, 6-3 to promising 62nd-ranked Croatian Borna Coric in a testing two hours, 51 minutes contest.

Coric tired towards the end and offered some openings to Granollers while missing three match points but was able to recover well to hold on and set up a meeting with sixth seed Dominic Thiem of Austria.

Russian 19-year-old Andrey Rublev made short work of world number 46 Florian Mayer of Germany, winning 6-1, 6-1 in just 44 minutes. Rublev will now face a tough challenge against 2010 Miami finalist and 10th seed Tomas Berdych.