SAN JOSE (Reuters) - Defending champion and top seed Fernando Verdasco held off the hard-hitting Ivo Karlovic 7-5 7-5 to advance to the quarter-finals of the San Jose Open Thursday. Also advancing with the Spaniard were second seeded Frenchman Gael Monfils, who outlasted Robert Kendrick 6-2 5-7 6-4, and fifth seeded Uzbek Denis Istomin, who edged American Michael Russell 7-6 7-6. Seventh-seeded Australian Lleyton Hewitt displayed his usual fighting spirit to rally for a 3-6 7-5 6-3 win over Brian Dabul, while Argentina's Juan Martin Del Potro continued his comeback from injury with a 6-1 7-6 win over Slovakia's Lukas Lacko. However, wild card Tim Smyczek caused the biggest upset of the day when the American battled to a 6-4 3-6 6-1 victory over Japanese eighth seed Kei Nishikori. Verdasco survived 14 aces by his Croatian opponent and broke once in each set to line up a last eight encounter with another hard hitter in the form of Istomin, who fired nine aces in his victory. Del Potro, the 2009 U.S. Open champion who is currently ranked 484th after struggling with a lingering wrist injury, is happy with his progress in just his third event of the year. "I'm serving very strong," Del Potro told reporters. "I'm happy to be in a tournament again after a year out. I'm improving my game slowly. That's important for the future and for the season too." Del Potro will face Hewitt, the former world number one and 2001 U.S. Open and 2002 Wimbledon champion. "It will be a tough match," Hewitt said. "He's a quality player and he's on the comeback from injury, so he hasn't played the most amount of tennis. But he's a quality player, and if he gets up, it's going to be a tough match." Smyczek, ranked 172nd, has fittingly advanced to his first quarter-final at the same tournament he qualified for his first tour event in 2008. He will face Monfils, who appeared to suffer a hand injury during his win over Kendrick. "It's obviously very exciting to be in my first tour-level quarter-final," Smyczek said. "San Jose was where I first qualified for a tour event, so it's always nice coming back here. I have some good feelings in this city."