LONDON, June 13: Top seed Rafael Nadal struggled into the quarter-finals of the Queen’s Club tournament on Thursday, beating Japanese teenager Kei Nishikori 6-4, 3-6, 6-3.

After his stunning demolition of the field at Roland Garros where he clinched a fourth consecutive French Open title without dropping a set, the Spaniard looked human after all against Nishikori on the slippery grasscourts in west London.

The king of clay seemed to have things under control until some uncharacteristic mistakes handed world number 113 Nishikori a break for a 5-3 lead in the second set.

Nadal quickly regained the initiative in the third set to ward off a shock but the Mallorcan will need to sharpen up in the quarter-final against Croatian ace-machine Ivo Karlovic who outgunned Chile’s Fernando Gonzalez 6-3, 7-5.

That match ended in unusual circumstances when Gonzalez, who had already been given a point penalty in the first set for smashing his racket, was docked the last game for ball abuse.

While Nadal showed he can still be fallible on grass, two of the surface’s finest exponents in recent years moved through to the last eight with few alarms.

Third seed Andy Roddick, the defending champion, needed just one set to get past Mardy Fish when his fellow American and close friend retired injured after losing a tight first set 7-6.

Lleyton Hewitt came back from 1-4 down in the first set against Frenchman Paul-Henri Mathieu to win 6-4, 6-4.

Results (prefix number denotes seeding):

Third round: 2-Novak Djokovic (Serbia) bt 13-Janko Tipsarevic (Serbia) 2-6, 6-1, 6-4; 1-Rafael Nadal (Spain) bt Kei Nishikori (Japan) 6-4, 3-6, 6-3; 4-David Nalbandian (Argentina) bt Nicolas Mahut (France) 6-4, 6-1; 11-Lleyton Hewitt (Australia) bt 7-Paul-Henri Mathieu (France) 6-4, 6-4; 6-Andy Murray (Britain) bt Ernests Gulbis (Latvia) 5-7, 6-1, 6-4; 5-Richard Gasquet (France) bt Mario Ancic (Croatia) 7-6(5), 6-4; 8-Ivo Karlovic (Croatia) bt 10-Fernando Gonzalez (Chile) 6-3, 7-5; 3-Andy Roddick (US) bt 15-Mardy Fish (US) 7-6(5) (Fish retired).