Six time former champion Roger Federer battled past the German prodigy Alexander Zverev in three tight sets on Tuesday night at the O2 Arena in London, while the American Jack Sock making his first appearance in London, stayed alive after coming from a set down to beat the unfortunate Croat Marin Cilic.

The second seeded Federer, who had a straight sets win over Sock on the opening day, assured himself of a berth in the semi finals on Saturday with a second win in the Boris Becker group, but was made to fight for his 7-6(6),5-7,6-1 win against the 6’6″ tall Zverev. The 20 year old German had beaten Marin Cilic also in three sets on opening day. Federer making his 15th appearance in London, is into the last four in London for the 14th time, the only time he missed out was in 2008 when the event was held in Shanghai. Last year he missed the event as he was out of action after a semi final loss at Wimbledon.

Games went with serve in the first set and after Zverev saved two set points in the twelveth game, he opened up a 4-0 lead in the enduing tie-break. However the Swiss won the next five points to take a 5-4 lead. Zverev had a set point at 6-5, but Federer won the next three to claim the first set. After an early break of serve, Federer dropped his serve for the first set time in two matches as Zverev levelled the second set 2-2. He broke once again in the twelveth game to send the match into a decider. Federer rose to the challenge and from 1-1 in the decider won the next five games breaking serve thrice. The match lasted two hours eleven minutes. Federer saved 9 out of 11 break points, and converted on four out of the eight break point chances.

Jack Sock, the lone American in the elite eight man field, came from a set down to beat the fifth seeded Marin Cilic 5-7,6-2,7-6(4) and improved to 1-1 in the Boris Becker group. Cilic, the 2014 U.S. Open champion, who had lost his opening match to Zverev also in three sets, fell to 0-2. Cilic, is the only player apart from Federer and Nadal in the elite eight to have won a grand slam title. The two hour twenty seven minute match was too close to call, Sock won 103 points, while Cilic won 102 points. Sock now faces Zverev in the third and final group match on Thursday, with the winner guaranteed to advance to a first ever appearance in the last four on to their debut appearance in London.

After his loss, Cilic is out of the tournament, in three appearances at the season finale he is 1-7 in London. On Sunday he lost 4-6,6-3,3-6 to Zverev and is now out of the running for a place in the last four. In the Pete Sampras group, Grigor Dimitrov wasted little time in beating the Belgian David Goffin 6-0,6-2 in only one hour and fourteen minutes in the afternoon session on Wednesday.

Dimitrov won 66 points while, Goffin won only 38. Both players are making their first appearance in the season finale, and indeed are the first from their respective countries to play the event. The Bulgarian Dimitrov is now assured of a place in the last four after improving to 2-0 this week. Goffin, who beat Rafael Nadal in his first match on Monday, now faces the Austrian Dominic Thiem on Friday, while Dimitrov will play the Spaniard Pablo Carreno Busta who replaced Nadal in the eight man field. Nadal, who assured of finishing the world number one ranked player for the fourth time in his career, withdrew from the event after his three set loss to Goffin. Busta, who replaces his compatriot as the first alternate player in the tournament also makes his debut in the season finale.

In the evening session on Wednesday, the fourth seeded Austrian Dominic Thiem took two hours and seven minutes to get past the Spaniard Pablo Carreno Busta, 6-3,3-6,6-4 and next faces David Goffin on Friday with the winner assured of a berth in the last four. Busta, came in as the first alternate after compatriot Nadal withdrew from the tournament due to knee injury. Thiem had lost his opening match against Dimitrov on Monday in three tight sets. The winner of Thiem v. Goffin match joins Dimitrov as the two semi finalists from the Pete Sampras group. Busta who had to win both his matches to qualify is eliminated but will have the chance to record his first win at the season finale when he faces Dimitrov on Friday.

– Rasesh Mehta [Rasesh Mehta is an analyst with the Tennis Galaxy. You can reach him at the email: sportzcosmos@gmail.com]