Kiefer saved two match points before the eighth-seeded Nalbandian served out the match with an ace to secure a third matchup this month against the ninth-seeded Del Potro. Nalbandian beat Del Potro in the semifinals of the Swiss Indoors last week after losing to him in the third round of the Madrid Masters the week before.
"This is going to be the third time in three weeks, so that is a little bit strange,'' Nalbandian said of facing del Potro again. "It's going to be a tough one, we both know each other very well.''
Nalbandian saved a break point in the sixth game of the first set but was then largely in control and it was Kiefer who struggled on second serve, fending off two break points in the ninth game.
Nalbandian broke him in the second game of the second set with a backhand shot that bounced on the line.
Meanwhile, tenth-seeded Gilles Simon advanced 6-3, 7-5 past Igor Andreev to keep alive his chances of reaching the season-ending Masters Cup in Shanghai, which is reserved for the top eight players in the ATP race.
Simon broke Andreev in 11th game to lead 6-5 and won on his second match point when Andreev returned into the net. The Frenchman next faces seventh-seeded Andy Roddick of the United States, who is also vying for a Masters Cup spot.
Simon and Roddick both took a step closer to qualifying after No. 5 David Ferrer of Spain lost 6-3, 6-2 to Germany's Philipp Kohlschreiber in Wednesday's play.
Ferrer started the week in ninth place in the ATP race behind Simon, del Potro and Roddick, and the Spaniard would now need a forfeit from another player to qualify for the prestigious tournament.
Marin Cilic of Croatia also advanced to the third round with a 6-4, 7-6 (2) win over Marcel Granollers of Spain and meets the winner of Wednesday's later match between Roger Federer and Robin Soderling.
Top-ranked Rafael Nadal of Spain was also in action later against Florent Serra of France, and fourth-seeded Andy Murray faced Sam Querrey of the United States.
The 21-year-old Murray is bidding to win a third straight Masters Series title after victories in Cincinnati and Madrid. Murray has five titles this year and hasn't lost since Federer beat him in the U.S. Open final.
(jp)
Nalbandian beats Del Potro in Swiss Indoors semis
Eliseu Delighted With ‘Portuguese First’
Del Potro knocks Nalbandian out at Madrid
