Rafael Nadal was made to battle again at the Mercedes Cup on Friday, the Spaniard beating Bernard Tomic in three sets as his preparations for Wimbledon continued.

A 6-4 6-7 (6-8) 6-3 victory kept the top seed - who needed three sets in his previous match against Marcos Baghdatis - on course for a third title in Stuttgart, Nadal booking a semi-final clash with Gael Monfils.

After his disappointing exit to Novak Djokovic at the French Open, Nadal has made a cautious start to the grass-court season, but he showed plenty of fight to come through against the big-serving Tomic.

Nadal grabbed the only break of the opening set in the seventh game, converting at the second attempt to move ahead before taking the set three games later.

He had to survive break points against him in the final game, though, and that gave Tomic confidence at the start of the second set.

The Australian needed a tie-break to level the match, Nadal wasting a match point before Tomic brought parity to proceedings.

Yet Nadal quickly took control of the decider by winning the opening three games and will meet Monfils in the last four after preventing any late fightback from Tomic.

Frenchman Monfils served 17 aces on his way to a 7-5 3-6 6-3 triumph over Philipp Kohlschreiber.

Second seed Marin Cilic had to overcome a rain delay and early scare against German qualifier Mischa Zverev before he could book his semi-final place.

After an hour wait due to the inclement weather, Cilic found himself a set behind to the world number 502, but slowly forced his way back into the match, taking the second in a tie-break.

He needed another in the decider as the big-serving pair rained down the aces - sharing 31 in the match - before Cilic found a forehand passing shot to secure a 4-6 7-6 (7-5) 7-6 (7-5) win.

Eighth seed Viktor Troicki is next up for Cilic after cruising past Sam Groth 7-6 (7-3) 6-1.