MADRID, May 13 (Xinhua) -- Spanish number one Rafael Nadal scored straight sets on Thursday to win over American John Isner to reach the quarter-finals at the Madrid Open tennis tournament.

Nadal followed Wednesday's comfortable 6-4, 6-3 win over Alexandr Dolopolov with a much closer match against the number 13 seed.

He struggled with the American's big serve, but was never under pressure on his own service games, although he had to wait until the 12th game of the first set before making his own break to take the set 7-5.

Nadal was always in control in the second set and again a single break was enough to allow him to take that set 6-4 and seal the win 7-5, 6-4.

"There are never any easy games, but I think I played better than in my last game and it is important to reach the quarter- finals," he told Spanish television station RTVE directly after the end of the match.

Fans in Madrid will not be able to look forward to a possible semi-finals between Nadal and fellow Spaniard, Fernando Verdasco, however.

The number six seed suffered a surprise straight sets defeat to Austrian player Jurgen Melzer, 7-5, 6-3 earlier in the day.