Hello and welcome to our LIVE coverage from the ATP World Tour Finals, where 17-time Grand Slam winner Roger Federer faces off against world number five Juan Martin Del Potro for a spot in the semi-finals.

So Roger Federer will face world number one Rafael Nadal in tomorrow's semi-finals after that impressive comeback win. And Novak Djokovic will play Stanislas Wawrinka no matter what the result in his match later on tonight against Richard Gasquet.

DEL POTRO 6-4 6-7 5-7 GAME, SET & MATCH; A booming forehand from Federer sets up the comfortable smash that brings up match point. And an ace seals it for the Swiss player. He is through to the semi-finals.

DEL POTRO - FEDERER 6-4 6-7 5-6 40-40; Federer drops the first point of the game before bouncing back with a great serve out wide that Del Potro can only put into the net. A netted forehand from Fed sees him slip behind once again before another great serve out wide has the same result as two points ago. 30-all. Del Potro flicks a low forehand across court to bring up an immediate break back point but he nets his backhand to squander the chance. Deuce.

DEL POTRO - FEDERER 6-4 6-7 5-6 BREAK; Federer finds an inside-out forehand winner across court to sneak ahead against the DelPo serve before the Argentine goes long with a forehand of his own to hand Federer two break points. It only takes the one though as another, slightly wild forehand, goes long from Del Potro and Federer will serve for a place in the semi-finals after the change of ends.

DEL POTRO - FEDERER 6-4 6-7 5-5: A great serve out wide sees Delpo go long but Federer can’t quite get his timing right and goes long on the next point. The Swiss takes the following point with a great little drop shot and another excellent serve sees him to 40-15 before Del Potro is the beneficiary of a double net cord to cut the arrears but then gets his timings wrong and launches high and wide.

DEL POTRO - FEDERER 6-4 6-7 5-4: What a way to open up the ninth game of the decider – Federer looks to have Delpo on the racks but he produces a tremendous “hotdog” lob to put the Swiss in trouble. Federer manages to come out on top in the end to lead 0-15. Delpo rattles off the next four points to take the game.

DEL POTRO - FEDERER 6-4 6-7 4-4: Federer replies in kind as he works his opponent this way and that to hold to 15. This looks to be going the distance – Nadal waiting for the winner tomorrow.

DEL POTRO - FEDERER 6-4 6-7 4-3: Delpo holds to 15 with ease.

DEL POTRO - FEDERER 6-4 6-7 3-3: Fed opens with an impudent flick of his wrist but Delpo is back in the game before a double gives the Argentine the advantage at 15-30. But two great serves gets the Swiss to game point, but he nets following some good defensive work from Delpo. Fed works another game point but some excellent baseline hitting from Delpo sees Fed net. It is advantage again after a netted forehand from Delpo and Fed then gets Delpo on the run before volleying his way to the game.

DEL POTRO - FEDERER 6-4 6-7 3-2: Dear me. Delpo sets up an easy-looking forehand for himself to get to 40-15 but goes long with Federer in no man’s land. Federer then comes out on top of an incredible rally to set up a break point. Delpo was in charge of that rally but goes long. Fed goes just wide with a forehand as he passes the break opportunity – deuce. Another rally ensues as Delpo again goes long. Incredible. Second break point. And Delpo goes long. BREAK!

DEL POTRO - FEDERER 6-4 6-7 3-1: Two lovely forehands sees Federer to a 30-0 advantage before Delpo goes long to give Fed the opportunity of a love game, which he duly takes.

DEL POTRO - FEDERER 6-4 6-7 3-0: The line judge calls a Delpo shot long when it was quite clearly in – Federer was on top in that rally and the Argentine aces the replayed point. Federer is furious and sends a backhand well wide as he struggles to control his emotions. An improvement from the Swiss sees him take a point but he then mis-hits a backhand before Delpo thuds a forehand pass by the racket of Fed.

DEL POTRO - FEDERER 6-4 6-7 2-0: Federer nets a volley having manufactured a point winning opportunity to give Delpo the advantage at 15-30. A smart serve out wide brings the game back to parity but Fed goes long with a forehand to give Delpo a break point, which he takes courtesy of a netted forehand from the Swiss.

DEL POTRO - FEDERER 6-4 6-7 1-0: Delpo thunders his way to a good hold to love in the deciding set to take the lead.

DEL POTRO - FEDERER 6-4 6-7: Federer gets a minibreak on Delpo’s second service point to lead 1-2 and then aces the Argentine before another booming serve sees Delpo net: 1-4. The Argentine sends an ace of his own thundering down to cut the arrears to 2-4. Delpo then goes wide with a volley to get the tie-break to 2-5. He can serve it out from here – and gets to 2-6 with a great forehand. ACE!!! Fed takes the second.

DEL POTRO - FEDERER 6-4 6-6: Some deceptive top spin from Federer sees the Argentine net but the Swiss passes up the opportunity to put pressure on the Delpo serve when he inexplicably drives a forehand long. Federer shows some strong defence to stay alive at the net before Delpo nets but a rasping forehand from the Argentine sees parity at 30-30. Federer nets on Delpo’s second serve – so disappointing from the Swiss’ point of view. Delpo stabs a backhand down the line to secure the game.

DEL POTRO - FEDERER 6-4 5-6: A poor attempt at a lob from Delpo this time allows Federer to smash his way to 30-0. And a tremendous forehand pass gets the Swiss to 40-0 before a serve out wide secures the game.

DEL POTRO - FEDERER 6-4 5-5: Delpo gets to 30-0 as Federer mistimes a lob volley after a great drop shot – Delpo nets to allow the Swiss back into the game. Delpo comes to the net to volley his way to two game points and then Federer can’t return a thunderous serve, so we are at 5-5.

DEL POTRO - FEDERER 6-4 4-5: Delpo takes the first point of the next game before a tremendous ace gets Federer to parity – he then comes out on top of a rally when Delpo nets to take the advantage. Delpo nets again to give Federer two game points – and he only needs one of those to take the game with another ace.

DEL POTRO - FEDERER 6-4 4-4: That’s the response the Argentine was looking for – he himself holds to love and we are at four games apiece. Federer had won 12 points on the spin prior to that.