For two guys who lost, Jo-Wilfried Tsonga and Rafael Nadal did their share of smiling at the Shanghai Rolex Masters on Saturday. Tsonga smiled and shook his head at his own mistakes; Nadal smiled and shook his head at his opponent’s lack of them. It wasn't all hits and giggles for Jo and Rafa, though; they also smacked their heads with their hands and punched their racquet strings. Neither man, it seemed, could quite believe what they were seeing.

And neither of them will play in the final tomorrow. Tsonga, who got off to his standard sluggish start and never fully recovered, lost 6-2, 7-5 to Novak Djokovic. A similar fate awaited Nadal in the second semi, except that it wasn’t his own slow start that doomed him to a 6-2, 6-4 defeat. It was the blazingly fast one by his opponent, Juan Martin del Potro. Rafa went down 0-4 and never caught up to the ball, or del Potro, today.

A final between Djokovic and del Potro shouldn’t be a surprise. Each won titles last week, neither has lost since the U.S. Open, and they have a history of finishing seasons well—I even picked this final myself before the tournament. Still, both of them were particularly good today.

Djokovic won 11 straight points to start, and he was up 3-0 in something approaching the blink of an eye. Jo was so discombobulated that he walked to the wrong side of the center hash mark to serve three different times. He was almost as out of it at the net; Tsonga won just 13 of 31 points up there. That was partly because of his own poor approach-shot choices; he insisted on going crosscourt to Djokovic’s forehand. But it was also because of Djokovic’s passing-shot brilliance. He dipped the ball at Jo’s feet, curled it by him crosscourt, and reflexed it down the line for winners.

Del Potro, if anything, was even better. I had wondered recently if he could still hit the titanic running crosscourt forehand that made him famous at the 2009 U.S. Open; I hadn’t seen him pull it off nearly as often in the three years since his comeback from wrist surgery. Today he unfurled something a little different, but just as scary: A down-the-line version of the shot in which he takes a deep and seemingly well-hit ball from his opponent, moves toward it, takes it earlier than normal, and fires it back harder than it came in. When del Potro rocketed one of those forehands past Nadal early in the match—he didn’t need to hit it anywhere near the lines—you knew he was feeling it today. The only question was: How long would the feeling last?

In their final in Indian Wells in March, del Potro had also won the first set against Nadal, only to lose in three. But there was no repeat today. Del Potro, who took a little off of his first serves and got 80 percent of them in, remained in control. Most impressive, perhaps, was how persistent he was in getting a break at the start of the second set, and not giving Rafa a chance to turn the match around. At 1-1, it took del Potro four break points, but he hung in long enough to win the game with a reflex volley winner. Nadal fought to keep it close after that, but he never had a chance to use the tactic that he likes to use against del Potro: High ground strokes to the big man's backhand side. Del Potro didn’t give him the time or the angle; he was too accurate to both corners with his forehand today. He saved all six break points he faced, the last one with a forehand that, for good measure, landed right on the baseline. Rafa, from what I could see, didn't smile at that one.

This was Nadal’s second loss on hard courts this year, as well as his second in two weeks. His late-season frustration continues—since 2005, he has just one title in an event played after the U.S. Open (Tokyo 2010). But as Rafa said later, there wasn’t much he could do one this evening. Del Potro, with his height, two-handed backhand, and ability to hit through the court, has all of the traditional ingredients needed to trouble Nadal, and a quick hard court in Shanghai didn’t hurt the big man, either. After the U.S. Open, Nadal had reassured the press, “Trust me, I will lose again.” Today he was even more realistic in his assessment. He admitted that he hadn't played badly, yet he had still lost 2 and 4. That’s how good del Potro was.

Del Potro was so good, in fact, that it didn’t look like he knew what to do when it was over. He finally just leapt in the air to celebrate. His reward? A date with Djokovic in the final. Their last match, a five-setter in the Wimbledon semis, was a classic. This one, while it’s not at a Grand Slam, could be just as consequential. Each of the last three years, a male player has used the fall as a jumping off point for a big start to the following season. Djokovic helped win the Davis Cup for Serbia in 2010, which catapulted him to his historic 2011. At the end of that year, Roger Federer won three tournaments in a row and was back at No. 1 by July 2012. Then it was Djokovic's turn to rise again. At the end of 2012, he won in Beijing, Shanghai, and at the World Tour Finals in London. Two months later he won the Australian Open.

It’s hard, as Djokovic found out in 2011 and Nadal is finding out again right now, for the player who dominates the Grand Slam section of the season to maintain the same level of motivation through the fall. Physically, you've played a lot of tennis; mentally, you must ask yourself, “Hey, what else do I have to prove this year?” That means there’s a chance for someone else to build momentum. So far Djokovic and del Potro have been taking that chance. We’ll see who keeps building tomorrow.