Let’s look at the top 8 seeds and see how they’re doing

Roger Federer

Federer had easy opponents the first two rounds.  Donald Young has been struggling to beat anyone this year, so he wasn’t ready to beat Federer.  Bjorn Phau is quick, but doesn’t have enough weapons to bother Federer.  Verdasco was his first tough opponent and still Verdasco of today is not Verdasco of two years ago.  Federer’s also never lost to him.  He has a walkover with Mardy Fish.  Federer’s looked sharp, but really hard to say until he plays someone that might bother him. And that someone could be Tomas Berdych.

Novak Djokovic

Djokovic has been having an easier time than Federer.  He also had two relatively easy opponents in Lorenzi (someone he crushed in Australia) and Dutra da Silva who rarely finds himself in the main draw of any event.  His third round opponent is the solid Julien Benneteau, but Djokovic made short work of him.  He’ll face his first interesting opponent in Stan Wawrinka who has been playing better this year.  Perhaps in the best form of the top 4.

Andy Murray

Murray’s had two good matches and two bad matches.  He played poorly against Alex Bogomolov, but broke him so many times, that it was still a rout.  He cleaned up his act in a night match against Dodig.  Dodig couldn’t really use his power much.  He struggled with the heat and Lopez’s power in the third round and nearly seemed out of it.  He came back with a sharp performance against Raonic where he returned and passed extremely well.  That win should give him lots of confidence.  Murray’s improving, but has had the toughest matches (in terms of winning) of the top 4 seeds.

David Ferrer

Has had maybe the toughest draw of the top 4 seeds, but keeps plugging along.  Beat Kevin Anderson in straight sets, then Sijsling in straight sets.  Hewitt pushed him to 4 sets.  Gasquet is up next, and could push the Spaniard, though Gasquet is so up-and-down, it’s hard to believe he’ll pull the upset.

Jo-Wilfried Tsonga

Got upset by Martin Klizan in the second round.  A bit of a surprise loss.

Tomas Berdych

No one much predicting he’d do well given an indifferent summer.  Did reach the finals of Winston-Salem the week before the Open.  Berdych also had a tough draw.  Opened against up-and-comer, David Goffin, had an easy opponent in Zopp the next round, then knocked out Querrey in four sets in the third round, and Almagro in the fourth round.  One of a handful of players that has given Federer trouble the last few years, and is starting to play some good tennis again.  Could make for an interesting quarterfinal.

Juan Martin del Potro

Slowly working his way up the ranks again, not nearly the meteoric rise he had from summer of 2008 to winning the US Open title in 2009.  Was expected to play Nalbandian in the first round, but veteran Argentine dropped out.  Replaced by Frenchman, Florent Serra.  del Potro still had an easy time.  Beat Harrison in four sets, then fellow Argentine, Leonardo Mayer in straights, though the third set went to a long tiebreak.  Playing Andy Roddick next.  In decently good form, but his match against Roddick will be key to seeing how well del Potro is really playing.  Roddick was the first really big upset del Potro had in 2008 when he knocked off the American in the finals of Los Angeles while everyone else was at the Olympics.

Janko Tipsarevic

Had a tough first round against Rufin, needing 5 sets to win.  Then, knocked out Baker and Zemlja in straight sets.  Kohlschreiber is a tough opponent who has been playing well as of late.  Tipsarevic probably lacks tools to go beyond quarterfinals.