Del Potro, seeded third, came out a 6-4, 7-5 winner over Beck. Del Potro dropped serve twice in the first set but didn't see a break point in the second when he lost just six points in six service games.
Even though the match was concluded in straight sets, it took nearly 2 hours. Del Potro's quarterfinal opponent Friday was on the court even longer as seventh-seeded Viktor Troicki needed 2 hours, 1 minute to defeat Jesse Huta Galung 7-6 (7-2), 6-3.
On the other end of the spectrum, top-seeded Roger Federer walked over to the quarterfinals when Mikhail Youzhny announced Wednesday a foot injury would keep him from playing Thursday. Also, No. 2-seeded Tomas Berdych was up 3-0 Thursday when Marcos Baghdatis retired with a leg injury.
No. 5-seeded Richard Gasquet posted a 6-3, 6-2 win over Alex Bogomolov Jr., and Andreas Seppi beat Philipp Kohlschreiber 6-4, 6-2. Both winners took 58 percent of the points in the match and had four breaks.
Federer faces Jarkko Nieminen in Friday's quarterfinals and Gasquet goes against Nikolay Davydenko in the top half of the draw. Berdych plays Seppi and del Potro is paired against Troicki in the other matches.