The 20-time Grand Slam champion has enjoyed a flawless start to 2018 winning the Australian Open and ABN Amro World Tennis Tournament in Rotterdam, and is bidding to claim a record sixth title - his 28th Masters 1000 crown - in the California desert.
"The question marks surrounding his defeat today will only grow larger", Courier said in commentary on the Tennis Channel. "I think it was a good match". "I made so many unforced errors that it was just one of those exdays where you are not able to find the rhythm from the baseline especially from the backhand side".
"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.
He is making a cautious return after an right elbow injury in 2017. "That is as head scratching as anything you'll see". The Croatian struggled to match Federer, who won 10 of the final 11 games in the match and at one point even won 17 straight points as he won the first set 6-2 and raced to a 3-0 lead in the second.
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.
Borna Coric of Croatia upset 13th seed Roberto Bautista Agut 6-1 6-3 and Spain's Pablo Carreno Busta defeated Daniil Medvedev 6-1 7-5.
"When you go from night to day [you are usually] playing a different opponent - so, naturally, it's all a change", said Federer.
"The night-time is much deader. The surface doesn't react so much".
Isner, the 15th seed, frittered away seven break points, while Monfils converted one of his three opportunities, which proved enough. "She's the best player I have ever played, and it was the biggest court I have ever played on".
In the next round he faces Jeremy Chardy, who beat fellow Frenchman Adrian Mannarino 7-5, 4-6, 6-1. I am on a good run right now and I want to maintain that.
Meanwhile, Kei Nishikori pulled out ahead of his opening match due to illness.