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Delray Beach, FL (Sports Network) - Former champion Tommy Haas advanced when his opponent retired Tuesday at the Delray Beach International Tennis Championships.

The second-seeded one-time world No. 2 Haas was tied with Igor Sijsling, 5-7, 6-4, when the Dutchman quit on Day 2 at Delray Beach Tennis Center.

Haas, who titled here back in 2006, will face Denis Istomin in the round of 16.

In matches involving Americans, Jack Sock whipped Aussie Matthew Ebden, 6-3, 6-3; Taiwan's Yen-Hsun Lu topped Bobby Reynolds 6-3, 7-5; Spaniard Daniel Munoz-De La Nava outlasted Ryan Harrison 6-2, 1-6, 6-3; and talented Latvian Ernests Gulbis dismissed an aging James Blake 6-1, 6-4. Gulbis titled here in 2010, while Blake was a Delray Beach runner-up in 2007 and 2008.

Additional opening-round wins came for Uzbekistan's Istomin, Frenchman Edouard Roger-Vasselin and Lithuanian lucky loser Ricardas Berankis, who assumed a spot in the draw when seventh-seeded Feliciano Lopez pulled out just two days after finishing as a runner-up in Memphis.

The 2013 champion will pocket $78,000.