* Q1 EPS $5.16 beats Wall Street view $4.93

* Q1 revenue $5.51 bln in line with Street view $5.53 bln

* CEO says economy tough, users searching but not buying

* Shares flat after initial 5 pct rise after hours (Adds YouTube, Twitter, analyst comment, CEO quote)

By Alexei Oreskovic and Anupreeta Das

SAN FRANCISCO, April 16 (Reuters) - Google Inc's (GOOG.O: Quote, Profile, Research) quarterly profit topped expectations, helped by cost controls, but Chief Executive Eric Schmidt said the economic environment remains tough with users still searching but buying less.

Shares of the No. 1 U.S. Internet search company initially rose 5 percent on the stronger-than-expected results, but then erased gains to trade flat after hours.

"We're still basically in uncharted territory," Schmidt said on a conference call. "Google is absolutely feeling the impact. Users are still searching but they're buying less. Ultimately, what that really means is the ads are converting less."

Compared to other Internet and media companies that depend on advertising revenue, Google has been extremely resilient to the economic downturn, though its revenue growth has slowed sharply from the heady 50 percent rates it used to enjoy.