Mexican reserve Esteban Gutierrez replaced compatriot Sergio Perez in first practice at the Indian Grand Prix today after Sauber said their regular Formula One race driver was unwell.A spokesman said a decision on the day's second practice session would be made later.Perez, who is joining McLaren next season while Gutierrez is a frontrunner to replace him in the race seat, cast some doubt on just how unwell he was with a comment on his Twitter feed.Gutierrez, 21, is due to test with Sauber in Abu Dhabi next month and, like 22-year-old Perez, is backed by Mexican telecoms company and team sponsor Telmex.