Ayrton Senna
1989: runner-up to Alain Prost

In 1989, the rivalry between Senna and Prost intensified into numerous battles on the track and a psychological war off it. Some controversy also arose after the French GP press conference when Ron Dennis declared that they found consistent differences between the Honda engines from Prost and Senna, to the detriment of Prost. Tension and mistrust between the two drivers increased when Senna overtook Prost at the restart of the San Marino Grand Prix, a move which Prost claimed violated a pre-race agreement (Senna denied the existence of any agreement, although Prost's story was backed up by John Hogan of the team's major sponsor, Marlboro). A discussion between the two drivers and Dennis during a test session at the Pembrey circuit in Wales served to effectively confirm Senna and Prost's personal animosity to Dennis and the team. Senna took an early lead in the championship with victories in San Marino, Monaco, and Mexico. Senna also led every lap of those races, a feat unequalled until Sebastian Vettel replicated it in 2012. Senna also managed to win in Germany, Belgium, and Spain. Unreliability in four consecutive races in Phoenix, Canada, France, and Britain, and further unreliability in Italy, together with collisions in Brazil and Portugal, swung the title in Prost's favour.

Prost took the 1989 world title after a collision with Senna at the Suzuka Circuit in Japan, the penultimate race of the season which Senna needed to win to remain in contention for the title. Prost had managed to leave the grid faster than Senna by removing the gurney flap from his car, which was unbeknownst to Senna. This reduction in aerodynamic downforce made Prost's car faster on the straights, but slower through corners—a clever choice to make it even harder for Senna to pass on a circuit already difficult on which to pass. On lap 46, Senna had finally come next to Prost and attempted a pass on the inside at the tight last chicane. Prost turned right into the upcoming corner, cutting Senna off and tangling wheels with him. The collision caused both McLarens to slide to a standstill into the escape road ahead. Prost abandoned the race at that point, whereas Senna urged marshals for a push-start, which he received, then proceeding with the race after a pit stop to replace the damaged nose on his car. He took the lead from the Benetton of Alessandro Nannini and went on to claim victory, only to be disqualified following a stewards meeting after the race. Senna was disqualified for receiving a push start, cutting the chicane after the collision with Prost, and for crossing into the pit lane entry which was not part of the track.

A large fine and temporary suspension of his FIA Super License followed in the winter of 1989, and an irate Senna engaged in public criticism of the FIA and its then-president, Jean-Marie Balestre, whom he blamed for his disqualification in Japan. Senna claimed that Balestre had forced the race stewards to disqualify him so his fellow Frenchman Prost could win the championship, though the stewards of the meeting denied that Balestre forced their decision, claiming that he was not present when the decision was made. Senna finished the season second with six wins and one second place. Prost, who could not stand working with Senna in what he felt was a hostile environment, left McLaren for rivals Ferrari the following year. Prost had burned bridges even with Ron Dennis after a trophy-related incident in Italy.
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