Alonso is toughest rival: Massa
"I've always had strong teammates but in the past few years the challenge has been even greater," Massa said Thursday, reports Xinhua.
"He (Alonso) is the most difficult driver I have come up against. Lewis Hamilton has also been a strong competitor as have Kimi Raikkonen and Michael Schumacher."
Massa and Alonso, both 31, have been teammates since the Spaniard's move to Ferrari from Renault in 2010.
Two-time world champion Alonso is currently leading the driver's standings with 179 points, 37 ahead of second placed Hamilton (McLaren-Mercedes). Massa is 10th overall with 47 points.
The next Grand Prix is in Singapore Sep 23.