UPDATE 1-Tennis-Teenager Harrison aims to extend dream debut

(Updates after Blake win)

HOUSTON, April 16 (Reuters) - Fifteen-year-old Ryan Harrison has set his sights on an upset victory over fellow American James Blake to keep his dream run alive at this week's U.S. Clay Court Championships.

The qualifier will face the world number nine in Thursday's second round, three days after joining Spaniard Rafael Nadal and Frenchman Richard Gasquet as the only players since 1990 to win ATP matches under the age of 16.

Top-seeded Blake won his opening match 6-4 6-4 against Japanese wildcard Kei Nishikori on Wednesday.

"I feel like I have a good chance," Harrison said of his prospects against Blake, a winner of 10 ATP career titles.

"If I play at the level that I played two days ago, then I have a chance for a win."

Texan resident Harrison stunned Uruguay's Pablo Cuevas 6-4 6-3 in Monday's first round, having lost just eight games in qualifying wins over Phillip King and Alex Reichel.

"The pressure is on them (the top players) to win against me I guess," added Harrison, replying to on-line questions on ESPN Sport's Web site (www.espn.go.com).

"There isn't a whole lot of pressure on me. I just go out there and play."