Rising American star Reilly Opelka: I'm a nightmare to play

Rising American star Reilly Opelka claims his game has improved a lot. The big-serving American continued his fine start to the season on Tuesday when he beat French Adrian Mannarino in the New York Open first round 2-6 7-6 (5) 6-4.

In January, Opelka made the Sydney International round-of-16 as a qualifier, before he stunned world No. 9 John Isner in the Australian Open first round. The 21-year-old rose to his career-high ranking of No. 89 in the world after making the semi-final at last week's Dallas Challenger.

"I've improved a lot in general. It may not look like the improvements are huge, but the little details add up so much. Serve's improved, the forehand is a lot better, understanding how to handle difficult moments in the match.

I'm a nightmare to play," Opelka said, per Newsday. "I've won a lot of three-set matches 

My court awareness is pretty high. Sitting down between the first and second set and being able to analyze that from sort of an outside perspective is huge." Opelka's next opponent in New York will be Uzbekistan's Denis Istomin, who beat USA's Mackenzie McDonald in the first round on Wednesday.

Opelka is expected to continue his rankings rise and achieve his new career-high ranking when the list gets updated next Monday.