Nishijima with a low kick in round three at a K-1 World Grand Prix 2009 in Tokyo Final 16 Qualifying GP superfight. With two good showings under his belt that year, Aerts then went up against Alistair Overeem at the K-1 World Grand Prix 2009 in Seoul Final 16 on 26 September 2009. Despite Overeem's youth, size and strength advantage, Aerts was still the favourite going into the fight as Overeem had primarily spent his career as an MMA fighter and not a kickboxer. Although there were no knockdowns in the match, Overeem dominated over three rounds and took the unanimous decision (30-27, 30–27, 30–27). After this fight, Aerts dropped more weight under a diet that Harinck set for him, dropping to 103 kg as opposed to his usual 109–112 kg. He took part in a tournament reserve bout at the K-1 World Grand Prix 2009 Final on 5 December 2009, defeating Gökhan Saki by unanimous decision (30-27, 29–27, 29–28). None of the tournament fighters were withdrawn, however, so this was the first time since the tournament's founding in 1993 that Aerts had not competed at the K-1 World Grand Prix. Aerts continued to drop weight, and on 3 April 2010, he faced K-1 Heavyweight (-100kg) Champion Kyotaro for the title at the K-1 World Grand Prix 2010 in Yokohama. He came into the fight at 97 kg. Despite looking in good shape, he was knocked down twice in the first round (although the second knockdown did not count as it came just as the bell sounded) before getting knocked out with one of Kyotaro's famous punches in the second. Following this defeat, Aerts regained some of the weight that he had lost for his match against dreadlocked karate man Ewerton Teixeira at the K-1 World Grand Prix 2010 in Seoul Final 16 on 2 October 2010. A close match, it was decided by an extra round unanimous decision (10-9, 10–9, 10–9) in favour of Aerts who then qualified for the K-1 World Grand Prix 2010 Final on 11 December. In his 17th appearance at the K-1 WGP, he defeated Mighty Mo by first-round KO in the quarter-finals. In the semi-finals, he came face to face with his archrival Semmy Schilt, in a fight where he was a huge underdog due to his age and Schilt's success in the 2009 Grand Prix. After being cut above the eye and finishing