Miami  - Rafael Nadal and Andy Murray were both pushed by aggressive opponents Monday before the pair of leading seeds constructed survival scenarios to reach the fourth round at the Miami Masters.

Number 1 Nadal had fought off a spirited challenge from Portuguese challenger Federico Gil for a tight, 7-5, 6-3 victory.

"No opponent is easy, especially if you are not playing your best tennis," said Nadal. "Everything was more difficult today. I didn't play very well, and I had chances to lose the first set."

Murray had a match point saved by a correct Hawk-eye challenge from Chile's Nicolas Massu, before the fourth-seeded Scot regained control moments later to finish off a 6-4, 6-4 decision in one and three-quarter hours.

"The end and the start weren't particularly good, but the middle part was very good," said the Scot after beating a second Argentine. "Obviously I'm happy I managed to close it out in two sets."

Nadal found himself in a surprise struggle with the unfancied world number 74 Gil.

The outsider recovered nicely from an early first-set break and pushed Nadal before dropping the set after losing serve in the 11th game.

Gil, 9-4 on the season, went up 2-0 in the second set, but Nadal again reeled in the challenger with a break in the first game before sprinting to safety.

"I didn't feel very well on court," Nadal said. "Sure, every match is very important for me. I want to win always, but you can't play well every day, and today was one of these days."

Nadal's victory was his 23rd of the season against just two defeats as he strives for a third 2009 trophy.

Murray and Nadal are showing the form that took them into the title match eight days ago at Indian Wells, California, where Nadal prevailed decisively.

They were joined in the last 16 in Miami by sixth seed Juan Del Potro as the Argentine beat Serb Janko Tipsarevic 6-4, 6-1. Viktor Troicki did better for the Balkan nation over German Bjorn Phau 6-4, 6-3.

In women's play, top seed Serena Williams kept her nose ahead as rivals nip at her current top ranking, defeating Zheng Zie of China 7-5, 5-7, 6-3.

Danish teenager Caroline Wozniacki stunned Olympic champion Elena Dementieva, sending the Russian fourth seed to a 7-5, 6-4 defeat.

"I'm just enjoying every day and every match that I'm playing," said the winner. "That I keep winning is just great."

Three-time champion Venus Williams showed her Miami pedigree as she fought off a late charge by Agnieszwa Radwanska to defeat the Polish 10th seed 4-6, 6-1, 6-4.

Williams almost let a set and 5-2 lead slip before making the escape.

"I just had a lot of errors, just errors, errors," complained the US fifth seed.

"Seemed like when I had the winner it became an error. On days like that, you just kind of have to let the errors go behind you and still continue to play aggressively and not pull back. Thankfully I know how to get through it."

Russian sixth seed Svetlana Kuznetsova beat compatriot Alisa Kleybanova 6-2, 6-1 to line up next against Wozniacki. Australian Samantha Stosur earned her third consecutive victory against former number 1 Amelie Mauresmo 6-4, 6-4. (dpa)