Nadal is trying to win his fourth Masters 1000 title of the year

Rafael Nadal and Novak Djokovic made swift progress into the quarter-finals of the Shanghai Masters as the event lost another two players to injury.

Top seed Nadal swept past Tommy Robredo 6-1 6-4 in the final match of the day, after world number four Djokovic had beaten Rainer Schuettler 6-4 6-2.

Nadal will next face Ivan Ljubicic, who led 11th seed Gael Monfils 6-2 3-0 when the Frenchman quit with a back injury.

Stanislas Wawrinka was 3-6 7-6 4-2 down to Radek Stepanek when he withdrew.

The Swiss was suffering with abdominal pains and joins Juan Martin del Potro, Andy Roddick and Tommy Haas as withdrawals during the week, while world number one Roger Federer (fatigue) and Britain's Andy Murray (wrist injury) had already skipped the tournament.

In Thursday's other matches, ninth seed Robin Soderling had a surprisingly comfortable

6-3 6-3 win over fifth seed Jo-Wilfried Tsonga and will next meet Feliciano Lopez, who defeated Jurgen Melzer 5-7 7-6 (7-3) 6-1.

Soderling is among a handful of players still in with a chance of claiming one of three remaining spots up for grabs at the ATP season-ending World Tour Finals in London next month.

"I know that if I play well, I have a chance, but this tournament won't decide who's going to play in London," said the Swede. "There's still a lot of tennis to be played."

Sixth seed Nikolay Davydenko beat 10th seed Fernando Gonzalez 6-3 7-5 to set up a clash with Stepanek.

Djokovic goes on to play eighth seed Gilles Simon, who beat Tomas Berdych 6-3 6-4.

"I've been working on physical strength and fitness a lot in last couple of months, and it's been paying off," said Djokovic.

"It's playing one of the key roles in the situations like this. When you are playing week after week, it's important to be physically strong, and that gives you advantage over your opponents."

Nadal was rarely troubled despite falling 2-0 behind in the second set against compatriot Robredo.

"I am playing well," said the Spaniard. "But I played two terrible games at the start of the second set so I have to improve in situations like this if I want to beat the top players."