Novak Djokovic moved a step closer to a record-equalling sixth ATP Finals crown with a 6-4, 6-1 win over Germany’s Alexander Zverev in London on Wednesday. 

The 31-year-old Serb is now unbeaten at the top of the Guga Keurten group with a 2-0 record. 

Djokovic, who struggled with flu symptoms during the match, broke Zverev late in the first set and twice in the second set to complete the victory in 76 minutes. 

“I don’t think it was breathtaking tennis, but a win is a win,” said the world number one. 

“He wasn’t close to his best in the second set so it looked a bit easier than expected because he served very well in the first set, but it dropped in the second set and allowed me to step in.” 

Croatia's Marin Cilic helped Djokovic advance to his eighth year-end semi-final through a permutation by conquering American John Isner 6-7 (2-7), 6-3, 6-4 in two hours and 14 minutes. 

“First set was extremely tough,” said Cilic after completing the tournament's first three-set match this week late in the evening. 

“John played a great tie-break so I had to stay patient, stay focused and I thought I played really great tennis from the second set. Really pleased with the win." 

Earlier on Wednesday, Zverev had the first two break point opportunities at 4-4 after Djokovic double-faulted, but the Wimbledon and US Open champion escaped with a drop shot to remain ahead. 

A double-fault from Zverev in the next game gifted Djokovic the set after 48 minutes. 

"Obviously I had chances in the first set," the 21-year-old world number five said.  

"If I break him at four-all, we'll see where the match goes. I think the match was much closer than the score says.

"Hopefully I still give myself the best chance to be in the semifinals."

The second set was much quicker as more unforced errors started to build to a match total of 36 for Zverev, and a long backhand conceded the first break for a 3-1 Djokovic lead. 

"I just started missing unnecessary balls," said Zverev. 

"Novak, when he's in the rallies, he's obviously very hard to beat. He's playing with a lot of confidence." 

Djokovic broke again for 5-1 with a determined forehand winner before brushing the baseline to open match point and concluded the contest with an overpowering delivery. 

Cilic erased three set points at 5-4 to remain level with Isner before heading into a first set tie-break. 

Isner surged 5-1 ahead in the breaker before securing the set with an aggressive forehand winner. 

A double-fault from the 33-year-old world number 10 gave Cilic a break for 5-3 before serving two straight aces to force a third set. 

In the first deciding set of the tournament, there were three consecutive breaks of serve with Cilic eventually holding serve for a 3-1 lead. 

The 30-year-old Croatian then served out his comeback triumph to claim only his second victory of the Tour Finals. 

His hopes of joining Djokovic in the semi-finals rest on how well he performs against the 14-time grand slam champion in the final round of group matches Friday. 

Djokovic leads their head-to-head record by 16-2 after winning their latest quarter-final meeting in three sets at the Paris Masters. 

"It was great match for us in Paris," said Cilic, who last defeated Djokovic at the Queen's Club final in June.

"I feel Novak is playing really good and I have to serve well, that’s going to be critical, and I hope to have another great match."

In the last round of Lleyton Hewitt-group matches on Thursday, Dominic Thiem faces Kei Nishikori before Roger Federer takes on his Wimbledon conquerer Kevin Anderson.