The American is the first 15-year-old in the Top 50 in nearly 15 years.
Defending Acapulco champion downplays bad blood with king of clay.
Former No. 1 passed six out of competition drug tests before launching comeback.
The Serb is making his first appearance in Dubai in four years.
The stacked draw features the former champions in opposite halves.
The Austrian missed a chance to surpass Roger Federer in the ATP rankings.
The South African had his second knee surgery in six months.
