Let's look at this matchup statistically. For the year Isner has won 20 matches and lost 11. His highest singles ranking for the year is 55. Isner just came off a 5 set victory over 5th seeded Andy Roddick, which is quite impressive. He's averaged 22 aces per match with a high of 38 against Roddick. He's averaged about 47 unforced errors per match and won 85 percent of his first serve points against Roddick.

http://www.usopen.org/en_US/players/overview/atpi186.html

Verdasco has won 39 matches and lost 16 this year. His highest singles rank for the year was 7th and currently is ranked 10th. Verdasco just came off a 5 set victory over 20th seeded Tommy Haas. He's averaged 13 aces per match, with a high of 18 against Haas. He's averaged 34 unforced errors per match, with 52 against Haas, while winning 72 percent of his first serves.

http://www.usopen.org/en_US/players/overview/atpv306.html

I'd say it's obvious that Isner has a big serve, averaging 9 more aces per match. His 3rd set victory over Roddick is more impressive than Verdasco's 5 set victory over Haas, a much lower seed in the tournament. For Isner to win, he'll have to continue his serving dominance, racking up aces and winning points on serve. Although Verdasco won't overpower him, but he doesn't commit as many unforced errors as Isner, which makes Isner's performance on serve all that much more important.

Will Isner win? I'd predict a close, hard fought match with Isner coming up a bit short, unless he totally dominates on serve. Just beating two top 10 seeds in succession is a tall order for Isner.

Verdasco in 5 sets, winning the 5th set 6-4.

Having said that, I hope i'm wrong and Isner takes the match.