Top-seeded Roddick crushed seventh seed Juan Monaco of Argentina 6-1 6-4 while fourth seed Isner recovered from a shaky start to beat eighth-seeded Cypriot Marcos Baghdatis 1-6, 6-3, 6-4.
Roddick enjoys a 3-1 advantage in career meetings with the towering Isner but he predicted a tough encounter in the last four with his six-foot nine-inch compatriot.
“It’s just whoever can scrape a return back on a big point and hope that something good happens,” Roddick told reporters after keeping his US Open preparations on a smooth winning track.
“The biggest thing is you’ve just got to try and take care of your own serve. I don’t often go into a match with the second-best serve but that will be the case tomorrow.”
Roddick, whose buildup to the season’s final grand slam had been interrupted by an abdominal injury, is bidding to reach his third ATP World Tour final of the year.
Isner was delighted to turn his match around against Baghdatis, who will be his first-round opponent at the US Open starting at Flushing Meadows on Monday.
“It was the second time I’ve played him this summer and I’ve had good success,” said Isner. “For sure I don’t give any players rhythm because on my serve I’m keeping the points short.
“A lot of the guys like rhythm and Marcos is one of those guys. When he gets into a groove from the baseline, that’s when he plays well and has good results.”
Ninth-seeded Ukrainian Sergiy Stakhovsky, who upset fifth seed Nikolay Davydenko of Russia in the previous round, ended his golden run on Thursday when he was beaten 3-6 7-6 6-2 by French qualifier Julien Benneteau.
Li, top seed Wozniacki advance
In New Haven, Connecticut, top ranked Caroline Wozniacki cruised into the next round and China’s Li Na had to endure a rain delay before booking her spot in the semifinals of the New Haven Open.
Wozniacki on Thursday remained undefeated advancing with a 7-5, 6-3 win over American teenager Christina McHale.
Three-time defending champ Wozniacki has a perfect 15-0 career record at the annual final tournament before the US Open, which begins in four days.
Wozniacki broke McHale’s serve in the eighth game of the second set of the one hour, 42 minute center court match.
China’s Li endured several hours of rain delays before advancing with a 6-4, 3-6, 6-2 quarter-final victory over Russia’s Anastasia Pavlyuchenkova.
Li will play Czech qualifier Petra Cetkovska who beat fourth-seed Marion Bartoli 7-5, 7-5.
Wozniacki will face third-seeded Francesca Schiavone, who advanced without hitting a shot after Anabel Madinah Garrigues withdrew due to a knee injury.
Madinah Garrigues, who was seeking a third title of 2011, fell to the ground and grabbed her knee after failing to convert a third match point in the second set of her 6-2, 7-6 (7/5) victory on Wednesday.
Caroline Wozniacki has a perfect 15-0 career record at the annual final tournament before the US Open Matthew Stockman, AFP/Getty Images
With the start of the US Open looming on Monday, Madinah Garrigues said she didn’t want to take a chance on making her knee worse.
She is the 30th seed for the final Grand Slam of the season and due to take on a qualifier in the opening round.
Even as they battled to get through the day’s matches, officials of the Open tune-up moved up the start of the championship match on Saturday in hopes of beating Hurricane Irene.
The tournament is scheduled to end on Saturday to allow players time to prepare for the US Open.
Organizers moved the start time of the final four hours to 1:00 p.m. (17:00 GMT) on Saturday.
Hurricane Irene, packing winds of up to 185 kilometers (115 miles) per hour, has already brought destruction to the Caribbean and Bahamas as it roars toward the East Coast of the United States.
Irene is expected to slam into North Carolina on Saturday and eventually to reach the New York area, which usually only experiences the remnants of hurricanes. New Haven organizers have already had to cope with Mother Nature this week.
On Tuesday, play was suspended for more than two hours as the earthquake with an epicenter in northern Virginia shook the stadium. Tuesday’s 5.8 magnitude quake was felt as far south as Alabama and as far north as Toronto.
