American youngster Ryan Harrison has revealed his ambition of becoming “the best tennis player in the world” after beating Grigor Dimitrov at the Abierto Mexicano Telcel.

The 22-year-old upset the defending champion 7-5 4-6 6-0 in Acapulco on Wednesday evening to reach his first ATP World Tour quarter-final of the year.

It was Harrison’s first win against a top-10 opponent in 23 attempts and sets up a last-eight clash with Croatia’s Ivo Karlovic.

Having defeated the world number 10, the youngster is expected to rise from 169th to around 134th in the rankings, but his long-term goal is much higher.

“I feel good,” Harrison told the ATP Tour’s official website. “I got into qualies (the qualifiers) as an alternate… and obviously here we are now. It’s crazy how things work.

“I’m just happy to be here. I’m really excited to focus on my career… I want to be the best tennis player in the world. It’s one of those things where you care about what you’re doing, you love what you’re doing and you want to be the best.”

To continue his run he will have to overcome sixth seed Karlovic after the big-serving Croatian came from behind to beat Dusan Lajovic 3-6 6-3 6-1.

It will be the first competitive meeting between the two players but Karlovic, who was crowned the Delray Beach Open champion last week, is the clear favourite.

Karlovic can be backed at 1.56 to win and progress to the quarter-finals while Harrison is priced at 2.44 to cause another upset.