Hello and welcome to live updates of the second semi final at this year's ATP World Tour finals. It's Grigor Dimitrov against Jack Sock in a battle of the debutants.

GAME, SET AND MATCH! - DIMITROV 4-6 6-0 6-3 SOCK. Dimitrov hits back from a set down to defeat Jack Sock and clinch his place in Sunday's final against David Goffin. The Bulgarian was edgy in what proved to be a sensational concluding game, blowing three match points and saving two break points before serving it out on his fourth. It was a tense one hour and 58 minute battle, but he eventually prevailed and will now end 2017 as the world No.3.

BREAK! - DIMITROV 4-6 6-0 5-3 SOCK. Dimitrov senses an opening with a sizzling backhand winner down the line, but Sock is having none of it at first. The American No.1 screams 'Come on!' as he battles hard to stay in it. Two double faults offer up break point at two different moments from deuce. Dimi unloads down the line and this time Sock can't stop him. Dimitrov will now serve for a place in the final.

DIMITROV 4-6 6-0 4-3 SOCK. That's Dimitrov's best service game of the set. He's been searching for the first serve on a more consistent basis and it finally fires here with a love hold.

DIMITROV 4-6 6-0 3-3 SOCK. No look-in for the Bulgarian. Sock finds more big serves, including his fastest of the night to easily hold to 15.

DIMITROV 4-6 6-0 3-2 SOCK. That was sublime! Arguably the best point of the match sees both players go hammer and tongs off either flank, with Dimi emerging triumphant with the deftest of drop volleys. It concluded a high quality game where Sock had again posed questions that Dimitrov struggled to answer.

DIMITROV 4-6 6-0 2-2 SOCK. No messing from Sock. He seems to be handling the pressure of this set the better of the two players thus far.

DIMITROV 4-6 6-0 2-1 SOCK. A big slice of luck for Dimitrov. He smacks the net cord with an attempt down the line, but it bobbles over to give him breathing space at 40-15. A double fault shows he's clearly feeling the pressure of the moment before a most welcome first serve keeps him in front on the board.

DIMITROV 4-6 6-0 1-1 SOCK. Dimi's seven-game winning streak comes to an end as a re-energised Sock gets on the board for the loss of one point.

DIMITROV 4-6 6-0 1-0 SOCK. Jack must have given himself a long look in the mirror during that bathroom break between sets. He comes out all guns blazing and earns an immediate break point. Dimi's first serve deserts him, but he unloads a couple of lovely forehands to edge ahead from deuce.

SET! - DIMITROV 4-6 6-0 SOCK. It's a bagel! Dimitrov has raised his level while Sock's has just dropped off a cliff. It's a third break meaning Set 2 lasted just 22 minutes. Dimi is level and has the bonus of serving first in the decider.

DIMITROV 4-6 5-0 SOCK. Dimitrov backs up the double break as a couple of crisp forehands and another unforced Sock error see him turn a 15-30 conundrum around. He's one away from taking us to a decider.

BREAK! - DIMITROV 4-6 4-0 SOCK. This time Dimitrov makes break points in game 4 count! Sock has gone away mentally at the moment. He may well see this set as pretty much over and save his energy for a decider now. There's an awful lot of work to do if he's to get back into it from here.

DIMITROV 4-6 3-0 SOCK. Dimitrov takes full advantage of the Sock dip to consolidate to 15. It's a similar story to how the early part of the first set went - can Dimitrov maintain his dominance this time?

BREAK! - DIMITROV 4-6 2-0 SOCK. The American's composed hot streak ends rather abruptly as a 30-0 lead turns pear-shaped. A serve-volley combo goes horribly wrong to offer up break point. If that was bad, then it got worse on the next point as a real shank off the backhand gifted it to Dimi.

DIMITROV 4-6 1-0 SOCK. Dimi gets the second set off to a positive opening with a routine hold to 15. He needs to shake off some of those nerves and loosen up. Easier said than done, of course.