Nadal recovered after dropping serve in the first set
World number two Rafael Nadal remains on course for a fourth consecutive Barcelona title after sweeping past Argentina's Juan Ignacio Chela.
The 21-year-old Spaniard fought back after dropping serve early on to win their quarter-final 6-4 6-2.
Nadal will play Denis Gremelmayr in the last four after the unseeded German beat Nicolas Amlagro 6-2 3-6 7-6 (9-7).
The other semi-final will see second seed David Ferrer of Spain take on Switzerland's Stanislas Wawrinka.
Ferrer beat compatriot Tommy Robredo 7-6 (7-4) 6-4, while Stanislas Wawrinka defeated another Spaniard, Albert Montanes, 6-3 6-7 (6-8) 6-4.
Nadal trailed 4-2 after dropping serve to love but Chela could not convert any of the five chances he had to consolidate the lead in the next game.
The top seed converted his third break point and broke again two games later, before going ahead at the start of the second set.
"If it had gone to 5-2 it would have been very difficult to win the first set," said Nadal.
"But getting back to 4-3 gave me a lot of confidence and I am very happy to be in the semi-finals for the fourth year in a row."