Sebastian Vettel believes Lotus is best placed to stop him winning a sixth straight grand prix in Abu Dhabi this weekend.
Grosjean arrives in Abu Dhabi fresh from three straight podium finishes, and having led much of the Japanese Grand Prix and come through from 17th on the grid to third in India a week ago.
Asked after practice who he saw as Red Bull's main threat, Vettel replied: "It will be the usual suspects, to be honest.
"Lotus is exceptionally quick, especially come Sunday, and may be able to pull off strong pace later on in the stint so we will have to keep an eye on them.
"Mercedes are strong on one lap for tomorrow."
Vettel admitted that he had no idea where Ferrari sat given its underwhelming Friday times. Fernando Alonso was only eighth in practice.
"I haven't seen much of Ferrari," said Vettel. "Judging the long-run pace is difficult."