Lewis Hamilton v Max Verstappen: Key moments in F1's thrilling title battle

Chaos, crashes and controversy - the 2021 Formula 1 season has had it all - and sets up a thrilling winner-takes-all finale between Lewis Hamilton and Max Verstappen.

It has been a roller-coaster year with the championship lead changing hands a number of times over the course of the 21 races, but Hamilton's win in Saudi Arabia last time out means the pair head to Abu Dhabi dead level.

BBC Sport takes a look back at some of the key moments of one of the most memorable seasons in Formula 1 history.

It was clear in pre-season that Red Bull had the pace advantage over Mercedes at some tracks and they took their first win of the season in the second race.

Both drivers displayed their credentials as the main title hopefuls, with Verstappen recovering from half a spin following a red flag to power off to a victory, while Hamilton overcame a rare error that saw him slide into the barriers at half-distance to finish second.

There was even a hint of what was to come in the tussle between the two when Verstappen forced Hamilton wide early on as the two raced side-by-side.

In June Verstappen was excelling again, storming his way to victory at the devilishly tricky street circuit in Azerbaijan before his tyre exploded on the 200mph straight, causing him to crash out.

After a resulting red flag, a feisty Hamilton looked set to take the lead at the restart, only for him to uncharacteristically knock a switch to activate his 'brake magic' function - which helps to keep heat in the discs at low speed - and cause his Mercedes to overshoot the first corner in a cloud of tyre smoke, eventually finishing a lowly 15th.

This was the race where the tensions in the title battle really took off.

Verstappen arrived at Silverstone in strong form after winning three races in a row, but Hamilton as in no mood to see his rival celebrate another victory in front of the Briton's home fans.

At Copse corner on the very first lap the two collided. Verstappen's race was ended immediately and there was even a trip to hospital for precautionary checks. Hamilton, who was blamed for the crash and given a 10-second time penalty, went on to claim a dramatic win.

Those in the Red Bull camp did not hold back with their criticism of Hamilton after the race, with Verstappen calling his celebrations on the podium "disrespectful" and "unsportsmanlike" while team principal Christian Horner said it was a "hollow victory".

Having started the British GP 32 points adrift of Verstappen, Hamilton finished it just seven behind. A seismic moment in the 2021 title battle.

After a blistering start to the season, disaster struck for Verstappen at the Hungarian Grand Prix - the final race before the summer break.

The first few laps of the race were some of the most dramatic seen in F1 for years as wet conditions resulted in a first-lap pile up in which Verstappen became tangled up in. Five cars were eliminated and although Verstappen was able to continue, his Red Bull was damaged and performance seriously compromised.

Nevertheless, things still looked OK for the Dutchman with Hamilton finding himself dead last after five laps because of a delayed pitstop, but the seven-time world champion fought back superbly to finish third and move into an eight-point lead at the top of the drivers' championship.

With tempers flaring after that Silverstone race, there was little surprise at another coming together between the two.

The second time this season the pair collided was at the Italian Grand Prix.

At this point, Verstappen's lead over Hamilton was five points but neither driver was able to pick up an advantage from this race as their crash led to both retiring.

The incident happened midway through the race when Hamilton came out of the pits alongside Verstappen and as the pair battled for position the two collided, with the rear of Verstappen's car landing on Hamilton's Mercedes.

Both retired, beached in the gravel. The Dutchman was deemed "predominantly to blame" and handed a three-place grid penalty by stewards for the next race.

Tensions were not just heightened between the drivers - there has been a war of words between the rival teams off the track for much of the season.

Mercedes team principal Toto Wolff and Red Bull counterpart Christian Horner have had plenty to say about each other's team.

Most recently, Red Bull raised concerns about the legality of the Mercedes rear wing, with Horner claiming it was giving Hamilton a speed boost over Verstappen - who was handed a 50,000 euro fine for handling Hamilton's car after qualifying in Brazil.

Earlier in the season, Mercedes had been wary of a similar phenomenon on the Red Bull.

Red Bull's concerns came at the same race Mercedes had requested a review of Verstappen's driving - during the Sao Paulo Grand Prix they believed Verstappen deliberately ran Hamilton off the circuit to avoid being passed, a review that was rejected by the FIA.

It was a race Hamilton went on to win after a blistering performance, in which he overcame two separate grid penalties, amounting to 25 places.

Nothing may have come by either team's objections - be it the car's legality or a driver's conduct - but the relationship between Red Bull and Mercedes has hit an all-time low in 2021.

The final flashpoint to get the title battle to a winner-takes-all finale happened just a few days ago - at the chaotic and controversial Saudi Arabian Grand Prix.

In one of the most dramatic grands prix in years, the race was stopped twice, featured three starts and had an extraordinary series of events between the title contenders.

But the most significant event happened in the final few laps when race leader Verstappen was told to give back a position to Hamilton that he had gained illegally.

Communication confusion resulted in Hamilton being caught off guard by Verstappen slowing down and he crashed into the back of the Red Bull, prompting Mercedes team principal Toto Wolff to slam his headphones to the floor in frustration as he watched from the pits.

Despite damage to his front wing, Hamilton was able to continue but Verstappen did not give back the position until six laps later.

Hamilton went on to win the race while Verstappen finished second despite being handed further time penalties, setting things up for Abu Dhabi.