On Saturday, the roles were reversed.
Federer, at 32, is on the verge of completing his worst season in over a decade; his win total (currently 45) will be his lowest since 2000, when he was 19. There was only one title, on the familiar grass in Halle, Germany. That Federer is hardly the dominant player he once was disappoints his supporters. But that is the reality of today's elite tennis when players are in their prime from the age of 25 to 30.
The Swiss champion, with 17 major trophies in his case at home, has gotten to the point where we cannot help but think he's playing well -- for his age. It's difficult to see him scuffle past his prime, but by all accounts, he is enjoying himself, surrounded by a growing family. He seems to revel in being Roger Federer, a status he has earned many times over.
When he fell behind del Potro love-3 in the third and deciding set -- an affliction he has struggled with all season long -- the six-time year-end tournament winner appeared to be done.
"I was probably slightly angry more than thinking it's going to be over soon," Federer said. "It's one of those moments today because I kind of fought back the whole match, the first, second set. Here we go again."
But then something marvelous -- and unexpected -- happened.
Federer rallied and schooled del Potro 4-6, 7-6 (2), 7-5 with some fiery tennis down the stretch. Federer summoned some steel resolve from the past and sprinted past the flummoxed 25-year-old. With the Argentine serving at 5-all, Federer crafted the decisive break when he deftly side-stepped to his left, even while the ball was going up, and cracked a forehand that del Potro couldn't return.
"Because of the last few weeks, probably I stayed relatively calm inside because I had a lot of ups and downs throughout the matches in Basel and in Paris, then again here," Federer said. "So for that reason I think that helped for me today not to worry too much. Okay, it's just a break.
"But I wasn't in many of Juan Martin's service games, so I kind of felt like probably I will get one more chance to break back. It's exactly what happened. Once on even terms, I was able to play a little bit more freely. For the first time, I was almost feeling like I was kind of in the lead."
And so, for the 11th time in 12 years, Federer is through to the semifinals of the year-end championship. That's an almost ridiculous level of consistency over time. Only Ivan Lendl (12) has more final four appearances.
Federer's reward? He draws No. 1 seed Rafael Nadal in Sunday's semifinals of the Barclays ATP World Tour Finals round-robin. No. 2 Novak Djokovic, working on a 19-match win streak, plays No. 7 Stanislas Wawrinka in the other session.
Federer is the oldest year-end semifinalist since Andre Agassi (33) in 2003. Agassi lost that final -- to Federer.
This was the third straight week that Federer and del Potro met. The Argentine won the Basel final and Federer took their quarterfinal at the Paris Masters last week.
"I think I got two chances to win the match," del Potro said. "I broke his serve in the second set and in the third one. But he also played great when I was up in the score, and he deserves to come back in both sets. In the end, when I had to be focused and find the winners, I made the mistakes."
Del Potro will finish No. 5 among ATP World Tour players for the first time since 2009, when he won his only major.
In the late match, No. 2 Novak Djokovic defeated No. 8 Richard Gasquet 7-6 (5), 4-6, 6-3. The final round-robin match had no bearing on the semifinal matchups.
Bryans into semifinals
The Bryans, at the age of 35, have recorded their best season ever in 2013 and have already clinched the year-end No. 1 ranking for the fifth straight year and the ninth overall.
Now the Sunday semifinals are set: The Bryans (2-1) will play No. 2 seeds Alexander Peya and Bruno Soares (2-1), while No. 3 Ivan Dodig and Marcelo Melo meet No. 6 David Marrero and Fernando Verdasco.
Forward spin: Day 7
After six days of round-robin play, we're down to the meat of this thing.
It says on paper that Djokovic has beaten Wawrinka no fewer than 13 times in a row. And certainly their most recent outing, a week ago in the Paris Masters, was a buttoned-down 6-1, 6-4 victory for Djokovic.
But in two meetings earlier this year in Grand Slams, they produced some phenomenal tennis, two of the best matches of 2013. In the fourth round of the Australian Open, Djokovic prevailed in five sets in a match that required 5 hours, 2 minutes. In the US Open semifinals -- Wawrinka's first major at that level -- it was five sets and 4 hours, 9 minutes.
This is Wawrinka's first year-end championship, and he has played credibly, winning two of three matches. He will be challenged to test Djokovic, who seems intent on meeting Nadal in the final.
Rafa, of course, faces Federer in the other semifinal -- for 32nd time in their careers. Nadal holds a 21-10 edge and has won all three of their matches this year and six of seven sets.