Novak Djokovic produced a puzzling performance as the former world No.1 lost to Japanese qualifier Taro Daniel in the second round at the BNP Paribas Open in California on Sunday.
On his return from right elbow surgery, Djokovic looked a pale shadow of his former self as he went down 7-6(3) 4-6 6-1 to make a quick exit from a tournament he has won five times.
"It felt like the first match I have ever played on the tour, very weird," he told reporters. "I just completely lost rhythm, everything, struggled a little bit with the health the last couple of weeks."
Djokovic took six months off after Wimbledon last year and returned in January's Australian Open where he lost in the fourth round. He had surgery in Switzerland shortly after that.
"I was grateful to be out on the court after surgery that quickly but at the same time just didn't feel good at all," he added. "Nerves were there. I made so many unforced errors."
Djokovic's performance in the final set against the world's 109th-ranked player on centre court raised some eyebrows, with four-times grand slam champion Jim Courier saying the Serb had "capitulated".
"The question marks surrounding his defeat today will only grow larger," Courier said in commentary on the Tennis Channel.
Holder Roger Federer avoided the same fate as Djokovic, though the Swiss top seed was pushed by left-handed Argentine Federico Delbonis in a match that was completed on Sunday after being halted by rain at 2-2 in the second set the previous day.
The top seed fended off a set point in the tiebreak to beat Delbonis 6-3 7-6 (8-6).
"It's been a long time since I have been interrupted at night and have to come back the next day," said Federer.
Meanwhile Australian Alex de Minaur had no answer to the heavy artillery of Juan Martin del Potro in their second round match, the big Argentine winning 6-2 6-1.
De Minaur, a wildcard in his first 1000 series event, had saved match points to win his opening round clash against German Jan-Lennard Struff.
But the sixth seed, who enjoyed an 18cm height and 28kg weight advantage, wrapped up the match comfortably in 68 minutes.
Frenchman Gael Monfils survived a match point before beating big-serving American John Isner 6-7 (7-5) 7-6 (7-3) 7-5 in a contest that had only one break.
Isner, the 15th seed, frittered away seven break points, while Monfils converted one of his three opportunities, which proved enough.
Yuki Bhambri of India pulled off a surprise when he toppled ninth-seeded Frenchman Lucas Pouille 6-4 6-4.
Israeli Dudi Sela beat British 21st seed Edmund 6-4 6-4, while Luxemburg's 24th seed Giles Muller was upset by Frenchman Pierre-Hugues Herbert 6-3 7-5.
In late action, Croat second seed Marin Cilic beat Hungarian Marton Fucsovics 7-5 6-3, while Canadian Milos Raonic defeated 17-year-old compatriot Felix Auger-Aliassime 6-4 6-4.
