REUTERS: Novak Djokovic produced a puzzling performance as the former world number one 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.
Djokovic's performance in the final set against the world's 109th-ranked player on centre court raised some eyebrows.
Four times grand slam champion American Jim Courier questioned the Serb's effort, saying Djokovic had "capitulated."
"The question marks surrounding his defeat today will only grow larger," Courier said in commentary on the Tennis Channel.
"We wondered post surgery what it was going to look like, how his elbow was going to react to the strain and stress, but it was his heart and head that didn't react the right way in this match. That is as head scratching as anything you'll see."
Twelve times grand slam champion Djokovic, who was seeded 10th, acknowledged his poor performance.
"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.
"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. Nerves were there. I made so many unforced errors."
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.
Daniel had won two rounds of qualifying and then held off fellow qualifier Cameron Norris to book a first meeting with Djokovic.
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 on Saturday.
Federer staved off a set point in the tiebreak to beat Delbonis 6-3 7-6(6).
Frenchman Gael Monfils survived a match point before beating big-serving American John Isner 6-7(5) 7-6(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 French ninth seed 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.
The tournament also lost Japan's Kei Nishikori, who withdrew through illness.
(Reporting by Andrew Both in Cary, North Carolina; Additional reporting by Gene Cherry)