The 32-year-old needs to reach the final of the season-ending showpiece to reclaim the number one spot from Nadal and match Pete Sampras's incredible record of finishing six separate seasons as the world's number one player.
The Swiss lost serve in the very first game after spraying a forehand error, pulling it back in the fourth game thanks to a forced mistake from the Austrian to level the score at 2-2.
Venez observer le passage de Mercure devant le soleil — Guillestre
Quel que soit le temps, des intervenants de l'observatoire expliqueront le phénomène des éclipses et des passages. En cas de mauvais temps d'autres images du transit de Mercure seront proposées et commentées.
"There's no other reasonable explanation (to that reaction) than trying to play as ideal as possible", Djokovic said from London. "Earlier this year I had a great stay in London, and over the years I've played well in this arena". "It was not easy for him to be playing his first Tour Finals match and I knew he might be a bit nervous".
"I was fortunate to get the break midway through the first set". Roger got broken at love in the 11th game following a deep return from Thiem who fended off a couple of break chances with good first serves in the next game to convert the second match point with a service victor that carried him over the top.
Hundreds of Indian Sikhs to make historic pilgrimage to Pakistan
But we never imagined we would ever be able to see it and have these feelings'. He said Pakistan and India can resolve the Kashmir dispute through dialogue.
Berrettini's forehand was responsible for 18 of those errors, including two shots into the net to hand Djokovic the two breaks of the first set.
Berrettini claimed a brilliant point in game two of the second set, managing to reach for a low backhand that Djokovic could only skew into the air, providing his opponent with a simple volley.
Another break for Djokovic followed, though Berrettini did capitalise on the only break point he was afforded.
Veterans Day 2019: List of discounts and giveaways offered for the holiday
Free concert with 4th Infantry Div Band from Fort Carson Where: Immanuel Lutheran Church, 46 East Pikes Peak Ave., at 3 p.m. Visit a Freddy's Frozen Custard & Steakburgers near you this Veterans Day, November 11, to get a free combo meal card.
But it merely stalled Djokovic's process for a brief moment - Berrettini, who is renowned for his powerful serves, going 2-0 and a break down.