US Open: Del Potro rues lack of killer instinct in US Open defeat
The Serb was in clinical form, retrieving Del Potro's scorching forehand time and time again and delivering a masterclass in defensive tennis to beat the Argentine.
"He played a great match, very smart game. I had my opportunities during the second and third set but I was playing almost at the limit all the time ... and I couldn't make it because Novak was there every time.
"He's a great champion. So I'm glad for him."
After losing an early break in the second set Del Potro fought back to level terms and then had three break points to open up a lead, but made two unforced errors that allowed Djokovic to see out the game and level at 4-4.
"I had my chances in the second set," Del Potro added. "I took the risks with my forehand. I've been doing that all the match. Sometimes it goes in and sometimes I miss it, but it's the only way to beat these kind of players.
"You have to play a perfect game during more than three hours. Sometimes you couldn't make it, but my mistakes were because of the level of Novak."
Federer won the Australian Open, Nadal won at Roland Garros, with Djokovic claiming the two final majors.
"I've been during all my career learning with Novak, Roger, Rafa, seeing them winning these events very often. It's amazing. I don't feel sad that I couldn't win Grand Slams because of them.