World number five Alexander Zverev says he was surprised Ivan Lendl decided to announce midway through the Hamburg Open that their partnership has ended.

The announcement came a few days after Zverev claimed eight-time Grand Slam winner Lendl talked more about "golf and his dog" than tennis.

The pair teamed up last year, with the 22-year-old German going on to win the season-ending ATP Finals in November.

"We both felt like we were going different directions," Zverev said.

"I have nothing but respect for him. We worked well for 10 months and it's been very nice having him by my side, winning London.

"It's also about personalities and maybe we didn't fit yet. Maybe it will change but for now it's over."

Lendl, who previously coached Britain's former world number one Andy Murray, said he still has a "lot of belief" that Zverev can fulfil his potential.

"I think that one day he may become a great player, but currently he has some off-court issues that make it difficult to work in a way that is consistent with my philosophy," Lendl said.

Despite the timing of the news, second seed Zverev has still reached the semi-finals of the Hamburg Open - his home tournament - after a gutsy comeback win against Serbia's Filip Krajinovic in the quarter-finals.

Zverev trailed by a set and a break before fighting back to win 2-6 7-5 6-2 in two hours and 20 minutes.

The world number five will play defending champion Nikoloz Basilashvili, who won 6-2 6-3 against France's Jeremy Chardy, in the last four.

Austrian top seed Dominic Thiem lost 7-6 (7-3) 7-6 (7-5) to Russian Andrey Rublev, while another upset saw Spain's Pablo Carreno Busta beat Italian third seed Fabio Fognini 3-6 6-2 7-6 (7-4).