The match resumed after rain suspended play 10 March.
Lock, who is on a tennis scholarship at UNLV, also revealed how the opportunity to train with the star trio came about and how it has inspired him ahead of his upcoming matches.
The Serb was once the most dominant player around - winning four straight grand slam titles from 2015 to 2016 - but little of that scintillating form was on display when the 12-time grand slam victor was upset by Japanese qualifier Taro Daniel 7-6 (7-3) 4-6 6-1 in the second round in Indian Wells on Sunday. Off to a fantastic start on the 2018 ATP World Tour, it appears he is the undisputed player to beat in men's singles right now.
"I just completely lost rhythm, everything", said Djokovic. He lost to Chung Hyeon in the Australian Open fourth round in January before undergoing the surgery. He had surgery in Switzerland shortly after that. "But I was grateful to be out on the court that quickly after surgery". His backhand, traditionally his most consistent and game-breaking weapon, bled errors-32 in all.
"The question marks surrounding his defeat today will only grow larger", Courier said in commentary on the Tennis Channel.
Djokovic had further surgery on a persistent elbow injury in February. "That is as head scratching as anything you'll see".
The score was deadlocked at 2-2 after 20-time grand slam champion Federer gracefully claimed the opening set 6-3 before rain wreaked havoc and suspended proceedings on Saturday.
While Federer completed a rain affected win over Argentine Federico Delbonis 6-3 7-6 (8-6) in his opener, Del Potro began his tournament by thumping promising Australian Alex de Minaur 6-2 6-1.
"But, you know, after an opponent like this, to come back from two breaks down, actually, for me it was nearly impossible". The surface doesn't react so much; the ball doesn't travel so fast.
Frenchman Gael Monfils survived a match point before beating big-serving American John Isner 6-7 (5-7), 7-6 (7-3), 7-5 in a contest that had only one break.
Other seeded players who lost were ninth-seeded Lucas Pouille, who fell 6-4, 6-4 to Yuki Bhambri; 14th-seeded Diego Schwartzman, 7-5, 6-4 to Marcos Baghdatis; 21st-seeded Kyle Edmund, 6-4, 6-4 to Dudi Sela; and No. 24 seed Giles Muller, 6-3, 7-5 to Pierre-Hugues Herbert.
They previously clashed in the final of the Buenos Aires Challenger a year ago, with Daniel rallying for a three-set win.
"But obviously having played only a couple matches in nine months, you are still in a way battling inside of your mind whether you are fit or not", Djokovic said.