Hello and welcome to LIVE updates of the ATP World Tour finals clash between Novak Djokovic and Alexander Zverev at the O2 Arena.

GAME, SET AND MATCH! - DJOKOVIC 6-4 6-1 ZVEREV. The world No.1 makes it two wins from two as he sets his sights on the semi finals. He serves it out to 15, cruising through a one-sided second set after a really tight opener.

BREAK! - DJOKOVIC 6-4 5-1 ZVEREV. Oh my, that's too easy. Djokovic offers a glimpse of those stunning return skills while Zverev continues to fall apart. A simple pass down the line to pick up an angled volley completes a break to love and will see the Serb serve for the match next.

DJOKOVIC 6-4 4-1 ZVEREV. The world No.1 easily consolidates as he clinches the love hold with a stonking forehand winner. After pushing Nole so hard for so long, Zverev looks to be fading fast.

BREAK! - DJOKOVIC 6-4 3-1 ZVEREV. That's a major blow for the German and could see this match come to a close rather sharply from here. He leaks errors off both flanks and is broken to 15, despite thumping down a ninth ace of the match. Djokovic is firmly in pole position now.

DJOKOVIC 6-4 2-1 ZVEREV. After the drama of the past few games, Nole maintains his early scoreboard advantage with a relatively routine hold to 15.

Meanwhile, news is breaking of a significant retirement in the women's game.

DJOKOVIC 6-4 1-1 ZVEREV. That's a big hold for Zverev. He comes under the cosh as a disguised dropshot and forehand pass put Djoker in the driving seat. The German recovers but is again pressed at deuce. Djokovic defends with all the elasticity he is famed for, but then drags a forehand crosscourt wide.

DJOKOVIC 6-4 1-0 ZVEREV. Ever the perfectionist, Novak screams in disappointment as he drills a crosscourt forehand wide. It gives Zverev a foothold in the game that is quickly swept away from him as the Serb's first ace of the contest gives him the platform to get Set 2 off to a strong start.

SET! - DJOKOVIC 6-4 ZVEREV. That'll hurt. Having gone so close to a break, Zverev comes unstuck under the scoreboard pressure and literally hands Novak a high quality opener in 48 minutes. The German leaks a series of errors, particularly with the crosscourt backhand and despite saving two set points, he double faults on the third.

DJOKOVIC 5-4 ZVEREV. Sascha's pass up the line gets a friendly tickle off the net and moves him to 0-30, but he gets impatient in the next rally and blows a backhand wide. The world No.5 thinks his big chance has gone, but then pushes Nole back to force a crosscourt error that hands him a break point. Djokovic lands a big first serve to make deuce but then double faults to offer up a second opportunity. Zverev battles hard in the next rally but sees his crosscourt lob land wide. Djoker takes full advantage of the reprieve to stand firm from there, claiming an important hold with a sublime drop shot.

DJOKOVIC 4-4 ZVEREV. A2Z continues to slam down some huge serves, picking up another pair of aces on his way to a dominant hold.

DJOKOVIC 4-3 ZVEREV. Sascha woos the crowd with a lovely pass up Djokovic's left wing and tries to up the ante. However, the Serbinator resists and takes the game to 30 with a wide serve the young German can only prod into the net on the stretch.

DJOKOVIC 3-3 ZVEREV. Some big serving from Zverev once again. A slight touch n the net 141mph serve denies him a fifth ace, but he follows up with a 140 mph once that Djoker can only block long. All square as we head into the business end of this set.