MIAMI – World number one Rafael Nadal held off a strong challenge from Portugal's Frederico Gil on Monday to win 7-5 6-3 and book a place in the fourth round of the Sony Ericsson Open.

Spaniard Nadal broke service twice in the first set and, after the determined and aggressive Gil broke early to go 2-0 up in the second, regained control to clinch victory.

Nadal will face the winner of the third-round tie between Russian Igor Andreev and Switzerland's Stanislas Wawrinka.

The Spaniard's powerful serve was the basis of his victory -- he produced five aces and won 76 percent of points on first service.

Nadal, however, was not satisfied with his performance.

"You can't play well every day, and today was one of these days," he told reporters.

"No opponent is easy, especially if you are not playing your best tennis. So everything was more difficult today."

British fourth seed Andy Murray won 6-4 6-4 against Chilean Nicolas Massu although he struggled at times against his 29-year-old opponent.

The Scot was 3-0 down in the first set before fighting back to 4-4 when Massu double-faulted on his serve at 30-30 and allowed Murray a vital break.

Murray produced his fifth break of the match to go 4-1 up in the second set but he labored in the final stages, allowing Massu back to 5-4.

The 21-year-old then saved two breaks points and double-faulted on his first match point before securing the win.

Murray will next face Serbia's Viktor Troicki, who beat German lucky loser Bjoern Phau 6-4 6-3 to reach the fourth round at Key Biscayne for the second year in a row.

Sixth-seeded Argentine Juan Martin del Potro booked his place in the next round with a 6-4 6-1 victory over Serbia's Janko Tipsarevic.