WIMBLEDON, England—Novak Djokovic led Juan Martin del Potro by a break in the fourth set and had two match points in a tiebreaker. But del Potro rallied from each deficit and has forced a deciding fifth set for a spot in the final. Thoughts after this latest turn of events:

—Djokovic is probably thinking about the two match points now, and will certainly be thinking about them more if he goes on to lose. He played both conservatively on both points, opting for cross-court shots and putting the ball on del Potro's racquet. The big man made Djokovic pay, eventually getting him out of position and eliciting a stab lob on the first match point, which del Potro "waved" out with his arms. The crowd, which del Potro has used to his advantage at times today, roared in approval.

—Del Potro was fortunate in one way on that match point: One of his early shots in the rally caught the tape, but safely made its passage to the other side of the net.

—Del Potro took the second match point in lesser time, striking a winner after dictating the terms.

—Del Potro's aggression resumed on his first set point. He got a look at a second serve and stepped into a big backhand return. Djokovic, even with his patented slide, could do nothing but put the ball into the net.

—Hawk-Eye played a big part in this set, most prominently when del Potro opted to not challenge one call after missing a couple challenges beforehand. Replays showed that del Potro's shot indeed caught the baseline, and it would have given him deuce while returning up 5-4.

—Another Hawk-Eye moment: Del Potro unleashed a laser forehand at one point, one of many he's hit so far (one was clocked at 113 M.P.H.). It was used, in my opinion, to actually make the call. His forehand was called out, but it was hit so hard that it's hard to believe anyone truthfully saw the spot where it landed.

—One more Hawk-Eye moment: Del Potro incorrectly challenged a call and stopped playing when down 1-0 in the tiebreaker. Down 2-0, del Potro wouldn't lead in the breaker again until 7-6.

—Djokovic broke serve to lead 4-3, but the way this match has played out, there's no way it was going to be smooth sailing, even with the advantage. Del Potro immediately broke back for 4-4.

Enjoy the fifth set, everyone. I'll have a report after the press conferences and Richard Pagliaro will have the Racquet Reaction.

Thoughts Through Three:

Novak Djokovic has just taken a two-sets-to-one lead over Juan Martin del Potro in today's first semifinal by winning a third-set tiebreaker. Djokovic won the first set 7-5; del Potro won the second 6-4. A few thoughts from the All England Club after three mostly exceptional sets of tennis:

—The tiebreaker, necessary after Djokovic and del Potro combined to hold serve from 15-30 down (or worse) six times in the 69-minute third set, ended with a reflex cross-court winner from the top seed. Before that, del Potro made two unforced errors. And before that, outright winners and unprovoked errors were uncommon. The great majority of the points played throughout this match have been earned through exacting physical play, with both men covering the court and transitioning from defense to offense in a pinch. It's been a contest of exceptional quality and has featured some jaw-dropping points.

—One of those came at 30-40 in the 12th game of the third set, after del Potro had just saved two set points. Forced wide by a Djokovic forehand, del Potro blasted one back cross-court on the run, which left the world No. 1 on the ground in an attempt to retrieve it. Djokovic has been on the ground often today, for what it's worth.

—Djokovic had a 0-40 lead in that game, and through three sets is one of 10 on break points. He let two slip away from 15-40 when leading 3-2 in the second; del Potro held and then earned his first break of the match for a 4-3 lead. The Argentine, who through three sets is one of four on break-point chances, would hold serve comfortably the rest of the way to level the match.

—It was a necessary set for del Potro to win, for up to that point Djokovic had been the only player getting pressure when returning. After del Potro held off various advances from Djokovic in the first set, he couldn't do it one final time when trying to force a tiebreak.

—Del Potro has served well overall and negated much of Djokovic's offense while returning serve. But Djokovic has served even better, though he was tested throughout the third set.

—It's a warm day in SW19 and the match is over three hours long already. Del Potro, who hyperextended his knee in his quarterfinal win over David Ferrer, doesn't look any worse for wear, but his condition bears watching.

—Del Potro has never beaten Djokovic at a Grand Slam tournament; he's 0-3 lifetime.

I'll be back with further updates should this head to a fifth set.