Sergio Perez admits he didn't expect Force India team mate Esteban Ocon to perform as well as he has in his first full year of F1.
The pair have been shadowing each other's moves since the 2017 season got underway, a close rivalry which resulted in some troubles over the summer when Perez and Ocon clashed with each other on several occasions.
Force India subsequently changed the rules of engagement, forbidding the pair to race each other, but the Silverstone-based outfit has kept both on board for 2018.
Mexican Perez told France's Auto Hebdo he was "surprised" by the French youngster's "level of performance" in 2017.
"I did not think he would have such results in his first year," said Perez.
"Even with a difficult car, Esteban still manages to go fast. That's one of his strengths and I have to learn from that," admitted the Mexican.
"Recently, he has also managed to improve a lot in qualifying, but for me it's good to have a teammate to push me to my limits. Because in today's F1, your teammate is your only reference.
"The three teams in front of us are untouchable," Perez, 27, added.
Perez, a veteran of 132 Grands Prix, currently enjoys a nine-point lead over his 21-year-old team mate in the drivers' standings.
Former McLaren man Jo Ramirez backs his countryman's view, and says Ocon's results could eventually land him a plum drive at Mercedes for 2019.
"If Sergio does not push his limits, Esteban will become the number 1," said Ramirez.
"The internal competition is good for Sergio, but he absolutely must succeed because this business is unforgiving," he added.
"Sometimes I wonder what Valtteri Bottas is doing at Mercedes, because he is clearly not at the same level as Hamilton. Especially when Esteban is also a Mercedes driver."
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