Righting the ship were Top 10-seeded winners No. 1 Rafael Nadal rolling Borna Coric 6-1, 6-2; No. 2 Roger Federer cruising past Canuck Peter Polansky 6-2, 6-1; No. 4 Alexander Zverev struggling past injured Richard Gasquet 6-3, 4-6, 7-6 (3); and No. 7 Grigor Dimitrov topping Mischa Zverev 6-3, 3-6, 6-3.
The 27-year-old Nishikori had two chances to serve out the match and led 6-2 in the decisive tiebreak, before Monfils won six points in a row to complete a superb fightback.
A rejuvenated Roger Federer believes he is still reaping the benefits of his extended break from tennis after a comfortable opening victory at the ATP Montreal Masters gave him his best start to a season since 2006.
Roger Federer eased back into action for the first time since winning his eighth Wimbledon title last month as he also reached the third round with a dominant victory. "It's pretty straightforward: trying to play on my terms".
Federer added: "I think it's just really learn from this week about how I need to play in Cincinnati and the US Open".More news: Andy Murray Out of Cincinnati With Hip Injury
The outburst appeared to steady the Swiss, whose serve and overall accuracy improved as the two-hour match progressed. The conditions were tough, with a lot of wind today.Sometimes it's hard to find the rhythm.
'But I was happy that I was very focused on my service games. I was aggressive. This is how I want to play for the whole week. Not disappointing his fans, Federer sealed his first win as a 36-year-old after holding his serve to 15, capturing the successful start to the tournament after just 54 minutes of play. He is competing in Montreal for the first time since falling to Jo-Wilfried Tsonga in the 2011 third round.
Milos Raonic who could have met Rafa Nadal in the quarter finals lost to French Adrian Mannarino in straight sets of 6-4 6-4.
Nisikori finished runner up a year ago to Novak Djokovic and had defeated Monfils in all three of their previous matches. He saved the first with an ace but Ferrer made no mistake with his second set point, converting to take the opener 6-4.