BARCELONA, May 8 (Reuters) - Nico Rosberg boosted his morale by keeping Mercedes team mate and championship leader Lewis Hamilton off the top of the timesheets in first practice for the Spanish Grand Prix on Friday.

Rosberg registered a best lap of one minute 26.828 seconds, 0.070 quicker than Hamilton, on a sunny morning at the Circuit de Catalunya as the sport kicked off its first European weekend of the season.

Hamilton, last year's winner in Spain, has won three of four races this season and started all on pole. He leads Rosberg by 27 points.

Ferrari's Sebastian Vettel was third fastest, but with a time of 1:27.806 that underlined the gap to make up on Mercedes, with team mate Kimi Raikkonen fourth in a predictable looking leaderboard.

Toro Rosso's rookie pairing of Carlos Sainz, making his home debut, and Max Verstappen were fifth and sixth respectively but Red Bull had another difficult morning with Australian Daniel Ricciardo sidelined for much of the session.

Ricciardo, who is already on his fourth engine for the season out of an allocation of four, did eventually leave the garage to set the ninth best time.

Susie Wolff made the third Friday practice appearance of her career with Williams, 40 years on from the only occasion on which a woman driver has finished in the points, and ended up with a faster time than two world champions.

The Scot, who is married to Mercedes motorsport head Toto Wolff, will not race on Sunday but was 14th fastest and ahead of McLaren's Fernando Alonso and Jenson Button -- 15th and 16th respectively.

Double champion Alonso was returning to the home circuit where he crashed heavily in pre-season testing, an accident that forced him to miss the Australian season-opener.

Wolff's best of 1:29.708 compared to Williams' race driver Felipe Massa's 1:28.831, although Wolff was carrying out a set programme with outright speed not the main focus. (Reporting by Alan Baldwin, editing by Sudipto Ganguly)