It quickly became clear at the start of the year that Mercedes would not have an easy ride to the title in 2021.
After seven years of facing no serious competition for either the drivers' or the constructors' title, the reigning world champions were suddenly on the back foot. Red Bull emerged as the team to beat in preseason testing as Mercedes struggled to adapt its 2020 car to regulation changes aimed at slowing the entire field to protect Pirelli's tyres.
The rule changes clearly impacted Mercedes' design concept more than its rivals, but that came with the bonus of creating one of the most competitive seasons to date as Red Bull finally got its car and engine package right from the first race.
Hamilton strikes the early blows
Despite Mercedes' early struggles, Hamilton still won the opening race in Bahrain before the two championship contenders started to trade victories in the first third of the season. Hamilton's largest points advantage came following a dominant win at the Spanish Grand Prix, but it proved be his last victory for 11 races and was followed up by his worst performance of the year at Monaco, where he finished a disappointing seventh despite his teammate Valtteri Bottas running in the top three before a pit stop issue.
A title-defining point for both drivers came at the Azerbaijan Grand Prix in Baku. Verstappen looked set for a comfortable victory for most of the race until his right rear tyre exploded at over 200 mph with just five laps remaining. The accident forced him to retire and resulted in the race being suspended while track workers cleared up the mess, presenting a straight fight between Verstappen's Red Bull teammate Sergio Perez and Hamilton for victory.
Hamilton made the better getaway at the restart, but left his car in the wrong brake setting ahead of the first corner and deep into the run-off, dropping him to the back of the field. Just as Verstappen had seen 25 points disappear in the blink of an eye with his tyre failure, so did Hamilton at the restart, with Perez going on to win the race.
Verstappen quickly put the disappointment of Baku behind him and then built up steam with an against-the-odds victory at the French Grand Prix and two dominant performances at Red Bull's home circuit in Austria. It looked as though Verstappen and Red Bull may have finally built the momentum to take a decisive lead in the championship, but then came the British Grand Prix at Silverstone.
The first major flashpoint
Such was the close nature of the battle between Verstappen and Hamilton that it seemed inevitable they would collide at some point in the season. The two had come close to taking each other out in Italy (Imola), Portugal and Spain, but Hamilton always backed out a the last minute to avoid a collision at the last moment.
At Silverstone Hamilton was the aggressor, coming from behind and needing to make a move stick in order to put a halt to Verstappen's championship momentum. After the two fought for position through the opening two thirds of the lap, it became increasingly clear that the battle would have to be decided either before, or in, the 190 mph Copse Corner. The sweeping right hander is just wide enough to accommodate two cars running wheel-to-wheel, but it requires impeccable judgement from both drivers as well as a willingness to leave each other space.
As Hamilton put his car up the inside, Verstappen showed no inclination of giving him extra space and as the front of Hamilton's car washed wide on a tighter angle, the cars tangled. In a fraction of a second, Verstappen's car was in a four-wheel slide and on course for a horrible crash.
The car stopped so suddenly against the barriers that 51 times the force of gravity was applied on both man and machine from 190 mph. Verstappen was lucky to walk away from the accident unaided, but he followed the circuit doctor's advice to go straight to hospital for further checks. Hamilton, meanwhile, continued on track and, despite a 10-second penalty for causing the accident, went on to win the race.
From that point onward the nature of the championship battle changed. Hamilton gained a much-needed 25-point swing in his favour in the standings, but Verstappen seemed more determined than ever to make the title his.
Collision two at Monza
The next time the two drivers found themselves in a wheel-to-wheel battle was on the opening lap of the Italian Grand Prix at the Roggia chicane. Fighting for second position, Verstappen once again left Hamilton no space, forcing the Mercedes across the run-off and dropping Hamilton down the order.
A slow pit stop for Verstappen later in the race put them back on a collision course at the first chicane on Lap 26, but this time neither driver was willing to give way. Hamilton, who had just emerged from his own pit stop, appeared to have claimed the position, but Verstappen left the nose of his car in the corner and the two made contact, pitching the Red Bull over the top of the Mercedes.
Verstappen's right rear tyre made contact with Hamilton's helmet as it passed over the Mercedes and only the halo cockpit protection on Hamilton's car prevented his head from being crushed. Both drivers retired from the race as a result of the collision, and once again the battle lines were dug deeper through comments to the media.
Hamilton finally took his 100th grand prix victory at the Russian Grand Prix in late September, reclaiming the lead of the championship while Verstappen finished second despite starting from the back of the grid due to an engine penalty. Hamilton took a new engine of his own in Turkey, dropping him 10 places on the grid, but could only recover to fifth as Verstappen took second place behind Bottas.
Hamilton wrestles back from the brink
But just as Mercedes looked set for a resurgence, Verstappen countered with back-to-back wins in the United States and Mexico. The Mexico win was particularly dominant and appeared to set Verstappen on course for title glory, with the form guide suggesting he would make it a hat-trick of victories at the next round in Brazil.
Verstappen's chances only seemed to strengthen when the rear wing on Hamilton's car failed a routine post session test in Friday qualifying and he was sent to the back of the grid for the sprint race on Saturday. But the disqualification seemed to light a fire under Hamilton, and armed with another fresh engine he carved his way through the field in both the sprint race and the grand prix to take a memorable victory on Sunday.
Chaos in Saudi
Hamilton built momentum with a dominant victory in Qatar, taking him to the penultimate round in Saudi Arabia with the chance to go level on points with Verstappen if he won the race ahead of the Red Bull. That's exactly what happened, but the way in which the two drivers got there was almost beyond belief. In a two-way battle for the lead in which Verstappen twice skipped Turn 2 trying to defend position and was twice told to give the position back, Hamilton eventually came out on top, but only after another controversial collision.
This one was the most bizarre of the three this season, as Verstappen slowed to let Hamilton past only for the Mercedes driver to slow up behind him for fear of giving Verstappen the DRS advantage on the following straight. The collision damaged Hamilton's front wing, and Verstappen was later blamed by the stewards after it emerged that he'd stood on the brakes directly ahead of Hamilton.
The result left them level on points ahead of the Abu Dhabi Grand Prix, but this remarkable season still had one last twist in the tale.
An incredible finish
The opening lap kicked off the controversy when Verstappen forced Hamilton wide at Turn 6 to briefly take the lead, before Hamilton cut Turn 7 and rejoined the track in the first place. Red Bull was livid, calling on Hamilton to give the position back, but the stewards determined no investigation was necessary, as any advantage Verstappen had gained by pushing Hamilton wide was negated by Hamilton skipping the second part of the chicane.
The Mercedes driver then extended his lead over the middle part of the race, with Verstappen's chances fading away as he struggled to match the Mercedes' pace. Hamilton looked locked on for an eighth world title before the situation was turned on its head on Lap 53, when Nicholas Latifi crashed his Williams under the W Hotel at Turn 14.
The accident brought out the Safety Car, allowing Verstappen to pit for new tyres while Hamilton stayed out on track. Red Bull had nothing to lose by making the late pit stop, whereas Hamilton risked giving away the lead when it was still unclear if the race would finish under the Safety Car.
Latifi's car was fully cleared by the penultimate lap, allowing racing to continue for one final lap. However, five lapped cars were on the road between Hamilton and Verstappen as a result of Verstappen's pit stop and there was limited time for the entire field to unlap itself as per the normal Safety Car restart rules.
Under pressure from Red Bull team boss Christian Horner, race director Michael Masi agreed to let the five cars unlap themselves while the rest of the field was told to remain in position. The decision was hugely controversial and had no precedent, leading Mercedes team boss Toto Wolff to unload his frustration to Masi over the team-to-FIA radio.
Despite Mercedes' anger at how the restart was dealt with, the race got back underway with a lap remaining and Verstappen made use of his fresher tyres to pass Hamilton for the lead into Turn 5. Hamilton fought back through Turns 6, 7 and 8 but didn't quite have the speed to attempt a move around the outside at Turn 9.
Verstappen went on to win the race and, with it, his first world title. It remains to be seen if the debate over the Safety Car period ends with the chequered flag, meaning F1's longest season in history may not be over yet.